Questions and Answers about COVID-19 Vaccines

As parents weigh the relative risk and benefits of getting their youngest children vaccinated against COVID-19, some wonder about the need for their child to get a relatively new vaccine when the disease doesn’t seem too bad in most children. Most healthcare providers agree that the benefits of vaccination outweigh the risks for our youngest family members:

• As of December 2022, about 1,500 children 17 years of age or younger have died from COVID-19. While this is a small number compared with the more than 1 million deaths in the U.S., for those approximately 1,500 families, their world will never be the same.
• Millions of children have been infected with the virus that causes COVID-19. Some of those children were hospitalized with severe disease or developed a condition called multi-inflammatory syndrome in children (MIS-C), which can damage organs and on rare occasions be deadly. Importantly, it appears that newer variants are less likely to cause MIS-C. Watch this video in which Dr. Offit discusses this trend.
• Like adults, some children who have had COVID-19, even mild cases, have experienced lingering symptoms, commonly referred to as “long COVID.” In younger children it may be difficult for them to express what they are feeling or experiencing, which can make this condition even more difficult to identify and address.
• Millions of vaccines have been administered safely to children at this point.

To see what others are saying, check out these videos that include clinicians, polio survivors, and families affected by influenza, another disease often thought to be “not too bad.”

Last updated: December 14, 2022

The Novavax COVID-19 vaccine uses a “tried and true” approach to inducing immunity. Specifically, the vaccine delivers the spike protein and an adjuvant, which is something that increases immune responses to the protein. It is given as two doses, separated by 3-8 weeks, to those 12 years of age and older. This technology is exactly the same as that used to make one of the influenza vaccines (FluBlok) and very similar to that used to make the hepatitis B and human papillomavirus vaccines.

To find out more about the Novavax vaccine, watch this video of VEC Director, Paul Offit, MD, who is on the FDA’s advisory committee and, therefore, reviewed the data presented during the advisory committee meeting.

Last updated: December 14, 2022

Many families are trying to determine whether to get their children boosted. Booster doses increase the antibodies circulating in the blood for about three to six months, decreasing the chance of any infection during that time.

Children 6 months to 18 years of age without immune-compromising conditions are recommended to get three doses of an age-appropriate mRNA-based COVID-19 vaccine. These children need all three doses to have the best protection. As of December 2022, the first two doses should be the monovalent mRNA versions and the third dose should be the bivalent mRNA version. It is preferable that all three doses be of the same brand (Pfizer or Moderna).

Children 6 months to 4 years of age with an immune-compromising condition should receive 3 doses of Pfizer or 4 doses of Moderna (age-appropriate versions) with the last dose being the bivalent version of the vaccine. Those 5 to 18 years of age with an immune-compromising condition should get 4 doses of an age-appropriate mRNA vaccine with the last dose being the bivalent version. It is preferable that all doses be of the same brand (Pfizer or Moderna).

For healthy children who have received the recommended primary doses of mRNA vaccine and were subsequently infected, a booster dose of vaccine is probably unnecessary. For children who have conditions that increase their risk of severe COVID-19, the additional dose may be of benefit.

For children who recently had COVID-19 disease, vaccination may be delayed for up to three months as they will be less susceptible to reinfection during that time and evidence suggests that they will develop a better immune response if there is more time between their illness and receipt of the vaccine. With this said, there is still a benefit to having children who were previously infected with COVID-19 vaccinated because the resulting immunity appears to be stronger than following either infection or vaccination alone.

Last updated: December 14, 2022

While myocarditis is rare, it is also real; so, we can understand why some parents may be hesitant to get their teens vaccinated. But it is important when making these decisions to realize that the choice not to vaccinate is also a choice to risk COVID-19, so let’s take a look.

Check out this infographic [PDF, 126KB] to see a visual representation of this information. Imagine that the entire population of Roanoke, Virginia, (approximate population of 100,000 people) was composed of 16- to 29-year-old males:

• If all of them were vaccinated, 5 of them would be expected to experience myocarditis following vaccination. (MMWR, Aug. 13, 2021 and NEJM, Dec. 2, 2021)
• If none of them were vaccinated, 960 would be expected to get COVID-19, 30 of those infected would be hospitalized, 6 of those hospitalized would end up in the ICU, and 59 would be expected to experience myocarditis caused by COVID-19. (MMWR, Aug. 13, 2021 and MMWR, Sept. 3, 2021)

The rates of myocarditis following vaccination of females are significantly lower. If Roanoke was filled with 16- to 29-year-old females, only 1 of the vaccinated group would be expected to experience myocarditis. On the other hand, if none of them were vaccinated, 1,280 would be expected to get COVID-19, 75 of those infected would be hospitalized, 5 of those hospitalized would end up in the ICU, and about 39 would experience myocarditis caused by COVID-19.

It is also important to realize that myocarditis following vaccination is short-lived and tends to resolve on its own, whereas myocarditis following an infection tends to be more severe.

Three other considerations are important when deciding about COVID-19 vaccination of teens (or teen athletes):

1. Data suggest that vaccination a few months after infection (at least 3 months) improves immunologic memory, so a previously infected, unvaccinated individual may be more likely to experience re-infection than a previously infected, vaccinated individual. And there is no evidence that myocarditis occurs more commonly following vaccination of those previously infected.

2. We are still learning about “long COVID,” the condition that causes people to experience symptoms well after their infection goes away. While we don’t yet know how often this occurs in younger people, it is clear that some young people suffer similar long-term consequences.

Click here to view a full-size image of the infographic [PDF, 126KB].

Last updated: December 14, 2022

In the U.S., the CDC recommends waiting until COVID-19 symptoms go away and the individual is no longer in isolation. They also indicate that while it is okay to get the COVID-19 vaccine or booster shortly after recovering from the disease, individuals may want to delay getting their vaccine or booster for about three months after onset of symptoms or positive COVID-19 test. Individuals are less likely to be infected again during this period, and evidence suggests that immune responses to the vaccine may be somewhat stronger. However, if an individual wants to get the vaccine sooner or if circumstances might preclude them from doing so later, they can proceed with vaccination once their isolation is complete.

Likewise, while the CDC previously recommended delaying vaccination for patients who were treated with antibody-based therapies, data now demonstrate that the modest reduction in antibody responses seen in these patients does not warrant the delay. With this said, most recently infected individuals are still recommended to wait for about three months before getting vaccinated, so the antibodies introduced by treatment are unlikely to be problematic anyway.

One stipulation that the CDC does recommend about antibody-based products relates to tixagevimab/cilgavimab (EVUSHELD™). If a patient recently received a COVID-19 vaccine, they should not receive tixagevimab/cilgavimab until two weeks after vaccination.

Last updated June 20, 2022; reviewed: December 14, 2022

The goal of vaccination is to prevent serious illness. This is achieved by generating immune memory cells, such as B cells and T cells. These cells are typically long-lived and reside in the bone marrow, bloodstream, and lymph glands to monitor for exposure to a pathogen. If the pathogen is detected, these memory cells quickly become activated and stimulate the immune response to efficiently fight the infection before the infection can get out of control and cause serious illness. In the case of COVID-19 mRNA vaccines, studies demonstrated that high levels of memory cells are generated, and as the delta and omicron variants emerged, we have seen that the levels of memory cells generated by both the mRNA (Pfizer and Moderna) and adenovirus-based (J&J/Janssen) vaccines have been sufficient to prevent serious illness in most cases. As such, these findings would not warrant a booster dose. 

However, a second goal of vaccination could be to prevent any level of illness, meaning that vaccinated people would not even experience mild or asymptomatic infection. To accomplish this, people need to have high levels of neutralizing antibodies circulating in their bloodstream. Neutralizing antibodies prevent the virus from attaching to and entering cells. Typically, neutralizing antibody levels fade over time. When this happens, a booster dose can stimulate the memory B and T cells to cause production of neutralizing antibodies, thereby increasing the level of detectable antibodies in the bloodstream and decreasing the chance for any level of illness for another brief period of time (a couple of months).

While prevention of any level of illness is a noble goal, historically, prevention of serious illness has been the goal of vaccination, particularly for respiratory infections, like COVID-19. These two goals have been at the heart of the scientific “debate” over the need for booster doses.

Watch a video of Dr. Offit discussing the decision to recommend booster doses.

• For a more detailed discussion of data related to booster dosing, you can view the free “Current Issues in Vaccines” webinar that Dr. Offit presented for healthcare providers on September 14, 2022. (Note: You will need to add your name, email address, state and country to view the recording, but we will not use your information. You can choose “no” for the email contact question. That is needed for our live event registration, as individuals are emailed reminders and the link. It will not apply for watching the recording. For other boxes enter “NA”.) The recording is free to view. Healthcare workers interested in free continuing education credits can get more information on this page of our website.)

Last updated: Jan. 20, 2022; reviewed: December 14, 2022

The language around booster dosing can be confusing. Those who have received two doses are considered “fully vaccinated,” and those who receive subsequent boosters are considered “up to date.” In the end, the mRNA vaccines are best considered as three-dose vaccines. Healthy young people less than 75 years of age can consider themselves protected against severe disease if they have either received three doses of the vaccine or two doses plus a natural infection. Yearly booster dosing should probably be reserved for those most likely to need protection against severe COVID-19 disease, specifically people more than 75 years of age, those who are immune compromised, and people with health problems that put them at high risk of serious illness.

J&J/Janssen adenovirus vector vaccine

People who received the J&J/Janssen vaccine should get a second dose of vaccine separated by at least eight weeks. The second dose should be a bivalent mRNA version.

Pfizer mRNA vaccine

No immune-compromising condition

• 6 months to 4 years of age: Three doses of age-appropriate product (maroon-colored cap). Doses one and two should be the monovalent version separated by three to eight weeks. Dose three should be the bivalent version separated by at least eight weeks from receipt of dose two. Children would not be considered immune until two weeks after receipt of the third dose. The dose for this age group is one-tenth that of the adult Pfizer dose.
• 5 years and older: All are recommended to get at least three doses of age-appropriate product (5- to 11-year-olds, orange cap; 12 and older, gray cap). Doses one and two should be the monovalent version separated by three to eight weeks. The third dose should be the bivalent version and should be given at least eight weeks (two months) after the second dose, preferably at least four months later. The dose for those 5 to 11 years of age is one-third that of the adult Pfizer dose.
• Those 5 and older who have not received the bivalent booster are recommended to get one dose of bivalent vaccine at least two months after receipt of the third dose.

Immune-compromising condition

• 6 months to 4 years of age: Three doses of age-appropriate product (maroon-colored cap). Doses one and two should be the monovalent version separated by three to eight weeks. Dose three should be the bivalent version separated by at least eight weeks from receipt of dose two. Children would not be considered immune until two weeks after receipt of the third dose. The dose for this age group is one-tenth that of the adult Pfizer dose.
• 5 years and older: All are recommended to get at least four doses of age-appropriate product (5- to 11-year-olds, orange cap; 12 and older, gray cap). Doses one and two should be separated by at least three weeks; doses two and three should be separated by at least four weeks. All three of these doses should use the monovalent product as they are considered primary doses for those with immune-compromising conditions. A fourth dose (booster) should be administered at least two months after the third dose and be the bivalent version. The dose for those 5 to 11 years of age is one-third that of the adult Pfizer dose.
• Those 5 and older who have not received the bivalent booster are recommended to get one dose of bivalent vaccine at least two months after receipt of the third dose.

Moderna mRNA vaccine

No immune-compromising condition

• 6 months to 17 years of age: Three doses of age-appropriate product should be received. Doses one and two should be of the monovalent version and separated by four to eight weeks. Dose three should be of the bivalent version and separated by at least eight weeks from receipt of dose two. These individuals would not be considered immune until two weeks after receipt of the third dose. The dose for this age group is one-fourth that of the adult Moderna dose.
• 18 years and older: All are recommended to get at least three doses. Doses one and two should be the monovalent version separated by four to eight weeks; these are both considered primary doses. A third dose (booster) should be given at least eight weeks (two months) after the second dose and should be the bivalent version. Booster doses of the Moderna vaccine should be half dose quantities compared with primary doses.
• Those 5 and older who have not received the bivalent booster are recommended to get one dose of bivalent vaccine at least two months after receipt of the third dose.

Immune-compromising condition

• 6 months and older: Four doses of age-appropriate product should be administered. The first three doses should be of the monovalent version. Doses one and two should be separated by at least four weeks, and doses two and three should be separated by at least four weeks. Children with immune-compromising conditions need all three doses to complete the primary series (i.e., none of these doses are considered booster doses). The fourth dose is a booster dose. It should be the bivalent version and be administered at least eight weeks (two months) after the third dose. These individuals would not be considered immune until two weeks after receipt of the third dose.
• Those 5 and older who have not received the bivalent booster are recommended to get one dose of bivalent vaccine at least two months after receipt of the third dose.

Novavax protein-based vaccine

No immune-compromising condition

• 12 years and older: All are recommended to get three doses of vaccine, with the third dose being a bivalent mRNA vaccine. The two doses of Novavax should be separated by at least three to eight weeks. The third dose, using an age-appropriate bivalent mRNA vaccine, should be given at least eight weeks (two months) after the second dose.

Immune-compromising condition

• 12 years and older: All are recommended to get three doses of vaccine, with the third dose being a bivalent mRNA vaccine. The two doses of Novavax should be separated by at least three weeks. The third dose, using an age-appropriate bivalent mRNA vaccine, should be given at least eight weeks (two months) after the second dose.

For an overview chart, see the CDC’s COVID-19 vaccine infographics:

• For people who are NOT moderately or severely immune compromised
• For people who are moderately or severely immune compromised

Immune-compromised individuals

People should talk with their healthcare providers to determine whether they are considered moderately or severely immune compromised since each individual is unique. However, the CDC has provided some guidance that may help.

People typically considered moderately or severely immune compromised include the following:

• People currently being treated for cancers of the blood or organs (so-called “solid tumor” cancers)
• People who received an organ transplant and take immunosuppressive medications to prevent rejection of the organ
• People who had a stem cell transplant or received CAR-T-cell therapy less than 2 years ago or who are taking immunosuppressive medications
• People with conditions that are considered to cause permanent immune deficiency because the condition affects cells of their immune system, such as DiGeorge syndrome or Wiskott-Aldrich syndrome
• People infected with HIV whose infection is untreated or considered to be at an advanced stage
• People currently being treated with one of the following types of medications:
  • High-dose corticosteroids (more than 20 mg prednisone or similar medications per day)
  • Alkylating agents
  • Antimetabolites
  • Transplant-related immunosuppressive medications
  • Cancer chemotherapeutic medications that are considered severely immunosuppressive (e.g., tumor-necrosis, or TNF, blockers)
  • Biologic agents that suppress or modulate the immune response

People who should work with their healthcare provider to determine their need for additional doses include:

• People taking medications that make them uncertain whether they would be included in the list of individuals mentioned above
• People with immune-system-related conditions not specifically mentioned above
• People preparing to start one of the above-mentioned medications

People not considered to be in this category include:

• People who do not have compromised immunity.
• People without a spleen.
• People who had cancer but are no longer being treated.
• People with chronic conditions that do not involve the immune system or require treatment with high doses of corticosteroids, such as diabetes, asthma, COPD, kidney disease, heart conditions, sickle cell disease, among others. If you are not sure, check with your healthcare provider.

Last updated: December 14, 2022

Yes. The CDC has indicated that people can get influenza vaccine and COVID-19 vaccine during the same visit, but in different locations, such as one in each arm or separated by at least one inch on the same arm.

Last updated: December 15, 2022

Yes. The CDC has indicated that COVID-19 vaccine can be administered at the same visit as any other vaccines. However, the vaccines should be given in different locations separated by at least one inch.

Last updated: December 15, 2022

The main difference between emergency use authorization, or EUA, and the normal process, which is via a biologic licensure application, or BLA, is how long data were collected prior to the vaccines being reviewed for use. So, when considered quite literally, the vaccines being used under EUA are no different than those that are used after the vaccines get full approval (BLA). The reason for the shortened timeline for COVID-19 vaccines was, of course, because of the pandemic. But, at this point, these vaccines have been given safely to millions of people and the companies have been monitoring vaccine recipients for more than a year.

Last updated: Jan. 20, 2022; reviewed: December 15, 2022

Even though the COVID-19 vaccines were initially released under Emergency Use Authorization (EUA), they were still approved by the Food and Drug Administration (FDA). The review process was the same, but because of the pandemic, the data could be submitted after a shorter period of participant follow-up than usual. However, even after submitting data (and getting an EUA), those studies continued. Pfizer’s vaccine has now been licensed for those 12 years of age and older, and Moderna’s, for those 18 years and older.

Last updated: July 21, 2022; reviewed: December 15, 2022

Cases of myocarditis, or inflammation of the heart, have been reported in a small number of people after receipt of the COVID-19 mRNA vaccine:

• The cases of myocarditis occur more often in boys and young men and more often after the second dose. Symptoms typically occur within 4 days after receipt of the dose. Recently immunized teens and young adults who experience chest pain or shortness of breath should be seen by a healthcare provider and report recent their vaccination.
• Myocarditis is somewhat common, particularly following viral infections. In fact, cases tend to occur more often in the spring due to viruses that circulate at this time of year (specifically, coxsackie B viruses). Typically, about 100-200 cases occur per million people per year.
• Available data suggest that the incidence of myocarditis following mRNA vaccines is about 1 per 50,000 vaccine recipients; however, this risk increases in males between 16 and 29 years of age to about 1 per 20,000 vaccine recipients. Of interest, myocarditis also occurs more commonly after either acute COVID-19 or as part of the multisystem inflammatory syndrome of children (MIS-C). For example, if 100,000 males between 16 and 29 years of age got the mRNA vaccine, about 5 would experience myocarditis. However, if 100,000 males between 16 and 29 years of age were infected with the virus that causes COVID-19, about 59 would experience myocarditis. These numbers are lower in females (See “My teen is a student-athlete and already had COVID-19, so does he need the COVID-19 vaccine?” for more detailed information or check this infographic.)
• Parents and teens should watch for symptoms that may include chest pain, pressure, heart palpitations, difficulty breathing after exercise or lying down, or excessive sweating. One or more of these symptoms may also be accompanied by tiredness, stomach pain, dizziness, fainting, unexplained swelling, or coughing. If a recently vaccinated teen develops these symptoms or you are unsure, contact the child’s doctor or seek more immediate medical assistance if needed.

Find out more in this article from our Vaccine Update newsletter for healthcare providers.

Watch a video featuring one of our pediatric cardiologists, Dr. Matt Elias, discussing treating patients with myocarditis.

Last updated: Jan. 20, 2022; reviewed: December 15, 2022

The mRNA vaccines are approved for those 6 months of age and older.

At this point, millions of children and teens have been safely vaccinated against COVID-19. The clinical trials in those 5 years of age and younger showed the vaccines to be safe and effective against severe disease. Moderna’s vaccine for the youngest children (6 months to 5 years of age) is one-fourth the dose (25 micrograms) of their adult vaccine (100 micrograms for 12 years and older). Children from 6 to 11 years of age receive a Moderna dose that is half the adult dose (50 micrograms). Pfizer’s vaccine (6 months to 4 years of age) is one-tenth the dose (3 micrograms) of their adult vaccine (30 micrograms for 12 and older). The Pfizer vaccine dose for 5- to 11-year-olds is one-third the adult dose (10 micrograms).

Bivalent booster dose versions have been approved for children as well:

• The Pfizer bivalent vaccines can be given to those 5 years and older, and they contain the same quantities of mRNA as the monovalent versions for each age group (10 micrograms for 5- to 11-year-olds and 30 micrograms for those 12 and older).
• The Moderna bivalent vaccines can be given to those 6 months of age and older, but they contain lesser quantities of mRNA than the monovalent versions of Moderna. Children 6 months to 5 years of age receive 10 micrograms. Those 6 to 11 years of age receive 25 micrograms, and those 12 and older receive 50 micrograms.

Watch the videos in the “Perspectives on COVID-19 Vaccine for Kids” to see why others support COVID-19 vaccination of children. Individuals featured in the videos include:

• CHOP clinicians caring for children with COVID-19
• Adults who survived polio as children and now live with the long-term effects of the virus, called post-polio syndrome
• Families affected by influenza, another respiratory virus that is often considered mild

Last updated: December 15, 2022

Since COVID-19 is still circulating and it takes several weeks for a person to be considered fully immunized, it is generally recommended to start the vaccination process with the vaccine the child is currently eligible to receive even if it is a lower dose.

If your child’s birthday occurs during the period between doses, the child will be offered the higher dose for their subsequent doses.

Last updated: June 21, 2022

Side effects in children were similar to what has been found in other age groups, including pain at the injection site, fatigue, headache, fever, chills, muscle pain, or joint pain.

Even though a small number of cases of myocarditis, or heart inflammation, have been identified in teens and young adults, particularly in the 4 days after receipt of the second dose of the vaccine, this side effect has not been found in younger age groups, who receive lower doses. However, it is still important to monitor younger children for this potential side effect. Chest pain, shortness of breath, or related symptoms should be reported to a healthcare provider.

Other serious side effects have not been identified, nor have long-term effects. Find additional information:

• Long-term Side Effects of COVID-19 Vaccine? What We Know.
• Reproductive Health and COVID-19 Vaccines

Last updated: June 21, 2022

While children and teens may not be as likely to get severely ill from COVID-19, it can still happen and, in fact, many children have been hospitalized with COVID-19. Most often, they have not been vaccinated.

With this in mind, parents trying to decide about vaccinating their children should consider the following:

• Conditions such as obesity, asthma, and developmental delay, as well as other pre-existing conditions, increase the chance for hospitalization.
• As of mid-December 2022, about 1,500 children and teens up to 17 years of age have died from COVID-19.
• As of late November 2022, more than 9,000 cases of multisystem inflammatory syndrome in children (MIS-C) have been diagnosed and 74 deaths occurred. MIS-C typically occurs 2 to 6 weeks after having COVID-19, can occur following a mild infection, tends to be more severe in adolescents and teens, and causes about 6 or 7 of every 10 individuals to be placed in intensive care. MIS-C can also affect heart function.
• We have yet to understand the lasting effects of infection, often referred to as “long COVID” in children.
• Finally, this age group can also transmit the infection to more vulnerable family and community members, such as those who are unable to get the vaccine.

Watch these videos, some of which feature CHOP clinicians describing caring for children with COVID-19, myocarditis, and MIS-C.

Last updated: December 23, 2022

No. The rumors related to COVID-19 vaccines affecting puberty or fertility are unfounded. The mRNA vaccines are processed near the injection site and activated immune system cells travel through the lymph system to nearby lymph nodes. In this manner, they are not traveling to other parts of the body. As such, there would not be a biological reason to expect that maturation or reproductive functionality of either males or females would be negatively affected by COVID-19 vaccination now or in years to follow. Importantly, due to reports of menstrual cycle changes following vaccination, studies have been, and continue to be, conducted. Early studies have suggested about a one-day difference in menstrual cycles; however, further data are needed to understand this finding and these reports, particularly because many factors can affect the timing of an individual’s cycle. As such, analyzing the data carefully will be important.

Watch this short video in which Dr. Paul Offit discusses COVID-19, the vaccines and infertility.

You can read more about fertility and COVID-19 vaccines in this Vaccine Update article.

Last updated: December 23, 2022

For individuals vaccinated in another country, they may or may not be recommended to get a COVID-19 vaccine in the U.S. based on the situation:

• If you received the recommended number of doses and boosters of a U.S.-approved vaccine (Pfizer, Moderna, Novavax or J&J/Janssen), you do not need to do anything. If you did not receive all recommended doses, you should follow the U.S. recommendations in accordance with your age and immune status. If you did not receive a bivalent booster, you should receive one booster dose if you are in a high-risk group, even if you had previously received a monovalent booster. If you received the same bivalent booster available in the U.S., you do not need to repeat the dose. If you received a bivalent mRNA vaccine that included the original SARS-CoV-2 strain and Omicron BA.1, you do not need a second bivalent booster.
• If you received the recommended number of doses and boosters of a WHO-approved vaccine or all recommended doses of a mix-match scenario that included all U.S.- or WHO-approved doses, you do not need to repeat the primary series. If you did not receive all recommended boosters (based on your age and immune status), you should follow the U.S. recommendations. You should receive a bivalent mRNA booster if you are eligible. This includes people who received a monovalent booster. If you received a bivalent mRNA vaccine that included the original SARS-CoV-2 strain and Omicron BA.1, you do not need a second bivalent booster
• If you got a vaccine that is not FDA- or WHO-approved, those doses should not be counted toward vaccination in the U.S. You should get the recommended number of doses of a U.S.-approved vaccine at least 28 days after your most recent dose of the non-approved vaccine. A preference has been expressed by the U.S. CDC to receive an mRNA (Pfizer or Moderna) or protein-based (Novavax) version. You should get a bivalent booster dose if eligible.

Last updated: December 23, 2022

Yes. Although minor changes (about one day in length) to the cycle have been observed, women do not need to schedule their COVID-19 vaccine around their menstrual cycle. The reasons for the changes are possibly the result of effects on specific types of immune system cells that are also present in the uterus or hormonal changes associated with the immune response.

Of note, the COVID-19 vaccine is not shed after vaccination, so being around recently vaccinated individuals would not be expected to affect someone’s cycle.

You can read more about menstruation and COVID-19 vaccines in this Vaccine Update article.

Last updated: December 23, 2022

Neither the mRNA (Moderna and Pfizer) nor adenovirus (J&J/Janssen and AstraZeneca) vaccines contain live virus. Each of these contain a single gene from the virus that causes COVID-19. The gene instructs our cells to make the protein, but no other proteins from the virus are made, so whole virus particles are never present. In this manner, people who were vaccinated cannot shed, or spread, the virus to other people as a result of vaccination. If, however, the individual subsequently becomes infected, they can spread the virus during the days before and early during their infection. Of note, the amount of virus shed by vaccinated people quickly decreases, so they generally shed less virus overall compared to unvaccinated, infected individuals.

The Novavax vaccine does not contain live virus, either. It delivers the spike protein directly, rather than having our cells make the protein. As such, viral shedding does not occur following receipt of this version.

Last updated June 21, 2022; reviewed: December 23, 2022

Viral shedding occurs when a person is infected with a virus and whole viral particles produced during the infection are transmitted in the individual’s secretions. For viruses that infect the respiratory tract, like COVID-19, these particles are often found in secretions from the nose and mouth, such as saliva or mucus.

Some people wonder whether they can shed the virus as a result of vaccination. In the case of COVID-19 mRNA and adenovirus-based vaccines approved for use in the U.S., the short answer is no. Both types of vaccines only introduce the gene for a single protein from the virus that causes COVID-19 – the spike protein. As such, whole viral particles are never produced during vaccine processing. Indeed, people are not considered to be infected when they are vaccinated because the virus does not replicate in them. Further, the vaccines are processed near the site of injection, so the spike protein produced during processing would not be found in nasal or oral secretions. As such, they cannot “shed” the single protein either. Likewise, the Novavax vaccine, which delivers the spike protein directly, cannot result in viral shedding.

However, if vaccinated people are infected, the virus will replicate at low levels in their nasal or oral cavity before the immune system stops it. In this scenario, the individual can shed the virus beginning about two days before the start of symptoms and through the first three to four days after symptoms begin.

Read more about viral shedding in this Parents PACK article, “Viral Shedding and COVID-19 — What Can and Can’t Happen."

Last updated: June 21, 2022; reviewed: December 23, 2022

People make mRNA all the time. In our cells, DNA in the nucleus is used to make mRNA, which is sent to the cytoplasm where it serves as a blueprint to make proteins. Most of the time, the proteins that are produced are needed to help our bodies function.

mRNA vaccines take advantage of this process by introducing the mRNA for an important protein from the virus that the vaccine is trying to protect against. In the case of COVID-19, the important protein is the spike protein of the SARS-CoV-2 virus. The mRNA that codes for the SARS-CoV-2 spike protein is taken up by cells called dendritic cells, which express the spike protein on the cell surface, travel to a local lymph node, and stimulate other cells of the immune system (B cells) to make antibodies. These antibodies protect us, so that if we are exposed to SARS-CoV-2 in the future, our immune system is ready and we don’t get sick.

The vaccine is processed over a 1- to 2-week period after vaccination during which time the immune response develops. However, the mRNA only directs protein production in the cell for 1 to 3 days before it breaks down. Once it breaks down, the cell stops making the spike protein.

• See more about dendritic cells and the adaptive immune system in this animation.
• Watch this short video of Dr. Offit describing how mRNA vaccines work.

Last updated July 29, 2021; reviewed: December 23, 2022

Adenovirus vector vaccines take advantage of a class of relatively harmless viruses, called adenoviruses. Some adenoviruses cause the common cold, but others can infect people without causing illness. To use these viruses for vaccine delivery, scientists choose types of adenovirus that do not cause illness and to which most people have not been exposed. They alter the virus by removing two of the genes that enable adenovirus to replicate in people, and they replace one of those genes with the one for the SARS-CoV-2 spike protein.

Like human cells, adenoviruses contain DNA as their genetic material. So, when an adenovirus vaccine is administered, it enters cells of the immune system called dendritic cells where it releases the DNA that includes the gene for the spike protein, and the genetic material enters the nucleus of the cell. In the nucleus, the DNA is used to make messenger RNA (mRNA), which is released into the cytoplasm to serve as a blueprint for making proteins. The DNA from the viral vector, however, cannot insert into the cell’s DNA. The mRNA causes the SARS-CoV-2 protein to be produced. The dendritic cells put pieces of the SARS-CoV-2 spike protein on their surface and travel to a draining lymph node where they stimulate other cells of the immune system; specifically, B cells that make antibodies, T cells that help B cells make antibodies, and other T cells that can kill virus-infected cells. Antibodies against the spike protein will now prevent the virus from causing an infection in the future.

Find out more about adenovirus vaccines in this Vaccine Update article, “Getting Familiar with COVID-19 Adenovirus-replication-deficient Vaccines.”

Last updated: Nov. 10, 2021; reviewed: December 23, 2022

The Novavax COVID-19 vaccine delivers the SARS-CoV-2 spike protein into our muscle. Once in our muscle, immune system cells that circulate throughout our body recognize the protein as foreign and attack it. Specialized immune system cells, called dendritic cells, put pieces of the protein on their surface and travel to nearby lymph nodes to activate other parts of the immune system. It takes about 1 to 2 weeks for the vaccine to be processed. The result is immunologic memory cells that are specialized to recognize the viral spike protein in the event of a future encounter with the virus.

This process takes advantage of our adaptive immune system, which responds to foreign proteins every day. To find out more about this part of our immune system, watch this animation.

Last updated: July 21, 2022; reviewed: December 23, 2022

In order for a virus to reproduce and cause infection, it must get into cells and take over the cellular machinery. Because viruses attach to cells using a particular protein on their surface, in this case the SARS-CoV-2 spike protein, scientists understood that blocking that attachment would be a direct way to prevent infection. One way to block this attachment is with antibodies that bind to the surface protein. As such, when the genome was published, scientists developing the nucleic acid or protein subunit vaccines (i.e., those that only used part of the virus) chose the gene for the spike protein, anticipating that this would be the most direct route to developing an effective vaccine.

Last updated: Mar. 31, 2021; reviewed: December 23, 2022

Most people are able to get COVID-19 vaccine. But, a few groups of people either should not get the vaccine or should get a particular version. Likewise, some individuals should consult with their doctor or follow special procedures.

People who should NOT get any COVID-19 vaccine:

• Those younger than 6 months of age.
• People currently isolating or experiencing symptoms of COVID-19; these people can get vaccinated once they are finished isolation and their primary symptoms have resolved although it is recommended that these individuals wait at least three months to be vaccinated so they develop a more robust immune response to the vaccine dose.

People who cannot get the mRNA vaccine (Pfizer or Moderna), but may be able to get the Novavax or J&J/Janssen vaccine:

• Anyone with a previous severe allergic reaction (i.e., one that causes anaphylaxis, any reaction that causes swelling that affects the airway (i.e., tongue, uvula, or larynx), or diffuse rash that also involves respiratory surfaces, such as Stevens-Johnson Syndrome) to a COVID-19 mRNA vaccine dose or an mRNA vaccine component.
• Anyone with a known allergy to polyethylene glycol.

People who cannot get the adenovirus vaccine (J&J/Janssen), but may be able to get the mRNA vaccine (Pfizer or Moderna) or protein-based vaccine (Novavax):

• Anyone with a previous severe allergic reaction (i.e., one that causes anaphylaxis, any reaction that causes swelling that affects the airway (i.e., tongue, uvula, or larynx), or diffuse rash that also involves respiratory surfaces, such as Stevens-Johnson Syndrome) to the COVID-19 adenovirus vaccine or one of its components.
• Anyone who experienced TTS following receipt of a previous dose of adenovirus-based COVID-19 vaccine.
• Individuals who experienced GBS within 6 weeks after receipt of an adenovirus-based vaccine or with a history of immune-mediated syndromes related to thrombosis or thrombocytopenia are not recommended to get adenovirus-based COVID-19 vaccines.
• Anyone with a known polysorbate allergy. (These individuals cannot get the Novavax vaccine either).
• Of note, the CDC has recommended that people get the mRNA version of COVID-19 vaccine when possible.

People who cannot get the protein-based vaccine (Novavax), but may be able to get the mRNA (Pfizer or Moderna) or adenovirus (J&J/Janssen) vaccine:

• Anyone with a previous severe allergic reaction (i.e., one that causes anaphylaxis), any reaction that causes swelling that affects the airway (i.e., tongue, uvula, or larynx), or diffuse rash that also involves respiratory surfaces, such as Stevens-Johnson Syndrome, to a COVID-19 protein-based vaccine (Novavax) dose or one of its components.
• Anyone with a known polysorbate allergy. (These individuals cannot get the J&J/Janssen vaccine either).

People who may get the vaccine after considering risks and benefits and/or consulting with their healthcare provider:

• Individuals with a history of severe or immediate allergic reaction to any vaccine or injectable medication or a non-severe, immediate (within 4 hours) allergic reaction to a previous dose of COVID-19 vaccine. (These individuals should be observed for 30 minutes after receipt of the vaccine.)
• People who have a severe or immediate allergic reaction to one of the types of vaccines and for whom the cause of the reaction is unknown (i.e., which component caused the reaction) should consult an allergist or immunologist to determine whether the individual can get the other version. If they proceed, they should be vaccinated at a location with medical facilities and staff prepared to respond to medical emergencies.
• People who cannot get one type of COVID-19 vaccine may be able to get the other type.
• People who are moderately or severely ill (regardless of whether they have a fever) may delay vaccination until they feel better.
• People with a history of MIS-C should delay vaccination until at least 90 days after diagnosis and they experience a return of normal cardiac function and are considered clinically recovered. Likewise, the risk for exposure should be high. Those with a history of MIS-A should be clinically recovered, including return to normal cardiac function.
• People who experienced myocarditis or pericarditis after receipt of mRNA or Novavax vaccine are typically advised not to get additional doses of any COVID-19 vaccine due to limited data about safety. In some instances, individuals and their healthcare providers may decide to proceed with an additional dose based on the risk-benefit assessment. Note: This does not apply to people with history of myocarditis or pericarditis unrelated to COVID-19 vaccination, nor does it apply to people with a history of heart disease.

People who should follow special procedures

• Someone with a history of severe or immediate allergic reaction (requiring medical intervention) to anything other than a vaccine or injectable medication can get the vaccine, but they should remain at the vaccination location for medical observation for 30 minutes after receipt of the vaccine.
• Pregnant women who develop a fever after vaccination should take acetaminophen. (See more in the pregnancy-related questions lower on this page.)
• People treated with convalescent plasma should not receive measles- or varicella-containing vaccines until at least 7 months after receipt of the plasma.
• People with a known COVID-19 exposure should wait until their quarantine is over before getting vaccinated. Unique situations may warrant vaccinating during this period, such as living in a situation with repeated exposures (e.g., living in a group setting, such as a nursing home, correctional facility, or homeless shelter) or having limited ability to be vaccinated after quarantine has ended.

Last updated: December 23, 2022

COVID-19 vaccines are generally widely available. As such, we recommend checking for vaccine at your provider’s office, local pharmacies, healthcare facilities, or mobile clinics. For children younger than 5 years of age, we recommend contacting your child’s healthcare provider or checking with clinics or pharmacies before going for vaccination as some may have certain age requirements for administering vaccines.

You can find your state’s information about COVID-19 vaccine distribution using this information prepared by our colleagues at Vaccinate Your Family.

Last updated: Nov. 10, 2021; reviewed: December 23, 2022

Common side effects from the mRNA, adenovirus and protein-based vaccines are caused as part of the immune response to each.

mRNA vaccines: Older children and adults 

The most common side effects from the mRNA vaccines (Pfizer and Moderna) are:

• Fatigue
• Headache
• Muscle aches

Side effects occurred during the first week after vaccination but were most likely one or two days after receipt of the vaccine. During clinical trials, side effects were more frequent following the second dose and more likely to be experienced by younger, rather than older, adults. Although most people will not have significant side effects, some people may wish to schedule their vaccination, so that they will not need to call out of work the next day if they don’t feel well.

A small number of people who get the mRNA vaccine experience mild, short-lived inflammation of the heart, called myocarditis. About 1 of every 50,000 mRNA vaccine recipients experience this condition, but it is most likely in adults 29 years and younger and more often occurs in males. This condition tends to occur within 4 days of receipt of the second dose, but can occur after any dose and up to several days after vaccination. Recently vaccinated individuals who experience chest pain or shortness of breath should seek medical care. This condition tends to resolve within 2-3 weeks and does not cause long-term heart damage. Importantly, COVID-19 infections can also cause myocarditis, and this tends to occur more frequently after infection compared with vaccination. (See “My teen is a student-athlete and already had COVID-19, so does he need the COVID-19 vaccine? We are worried about myocarditis.” on this page for more detailed information.)

mRNA vaccines: Children younger than 5 years of age

Young children who received either the Pfizer or Moderna mRNA vaccine commonly experienced:

• Pain, tenderness, and swelling near the injection site
• Fever
• Irritability
• Decreased appetite
• Fatigue

Older children in this age group, who are better able to communicate what they are feeling, sometimes also experienced headaches, chills, achiness or joint pain, and nausea or vomiting. These effects were somewhat more likely after receipt of the Moderna vaccine, which is a higher dose, but occurred infrequently overall.

Myocarditis was not detected in either clinical trial; however, because COVID-19 mRNA vaccines are a rare cause of myocarditis in older adolescents and young adults, it is possible that it could be observed in younger children. Experience with these vaccines in older children and adults suggest that the likelihood of myocarditis is significantly lower following vaccination compared with infection. Also, the doses given to this age group are even lower than those given to older children and adults. However, parents and care providers should still monitor their children in the days following vaccination and contact healthcare providers or seek emergency care should concerns arise.

Protein-based vaccine: Adults

The most common side effects from the protein-based vaccine (Novavax) are:

• Injection site pain and less often redness or swelling
• Headache
• Fatigue
• Muscle aches

A small number of cases of myocarditis have occurred in individuals who received this vaccine; however, additional data are necessary to determine the level of risk. Recently vaccinated individuals who have heart-related symptoms should seek medical care.

Adenovirus-based vaccine: Adults

The most common side effects from the adenovirus vaccine (Johnson & Johnson/Janssen) are:

• Injection site pain and less often redness or swelling
• Headache
• Fatigue
• Muscle aches
• Fever

Side effects occurred during the first seven to eight days after vaccination but were most likely to occur one or two days after receipt of the vaccine. Side effects were more often experienced by younger, rather than older vaccine recipients.

Two rare, but potentially dangerous conditions, have been identified following receipt of the adenovirus-based vaccines, such as the J&J/Janssen version:

• Thrombosis with thrombocytopenia syndrome, or TTS, occurs in about 1-2 of every 1 million vaccine recipients and develops up to 3 weeks after getting vaccinated. Individuals between 18 and 64 years of age, both female and male, who got the J&J/Janssen vaccine have experienced this condition; however, women between the ages of 30 and 49 years of age are at the greatest risk. Anyone who got the J&J/Janssen vaccine less than 3 weeks ago should seek medical care if they develop severe headache, shortness of breath, severe abdominal pain, unexplained leg pain, easy bruising, or small red spots on the skin. Anyone seeking medical care with one or more of these symptoms should mention their recent receipt of the vaccine, so healthcare providers can order the appropriate diagnostic tests and treatments.
• Guillain-Barré syndrome, or GBS, occurs in about 1 of every 100,000 vaccine recipients, most often during the first 3 weeks after getting vaccinated. The condition has most often been identified in males between 50 and 64 years of age, but it can occur in females and those 65 years and older on occasion. While rare, most cases have required hospitalization and at least one person has died. Anyone who recently received an adenovirus-based COVID-19 vaccine and experiences muscle weakness or paralysis should seek medical treatment and inform the healthcare provider of the recent vaccination. It should also be noted that COVID-19 infection has been associated with GBS; so, natural infection with SARS-CoV-2 also appears to be a rare cause of GBS. Find out more about GBS in this Parents PACK article, “Guillain-Barré Syndrome (GBS) & Vaccines: The Risks and Recommendations.”

Check out this infographic [PDF, 157KB] to see a visual representation of this information.

Last updated: July 21, 2022; reviewed: December 23, 2022

The CDC has indicated that you can take anti-fever or anti-inflammatory medications if necessary following COVID-19 vaccination, but it is important to know that doing so could diminish the level of immunity that develops. This is true anytime you take these types of medications, whether following vaccination or to treat illness. Generally speaking, the “symptoms” people experience following vaccination or during illness, such as fever, redness at the injection site, or fatigue, are caused by your immune system response. For example, fever is your body turning up its “thermostat” to make the immune system more efficient and the pathogen less efficient. For these reasons, if you are not very uncomfortable, it is better not to take these medications.

Some wonder how long they should wait after vaccination before taking these types of medicines, so their immune response is not affected. As a rule of thumb, the immune response following receipt of the mRNA vaccine develops over a week or two after vaccination and for the adenovirus vaccine, over the course of about four weeks, but the greatest chance of affecting your immune response would be in the first few days after receipt of the vaccine. Indeed, in the adenovirus vaccine studies, about 1 in 4 vaccine recipients took fever-reducing medication (antipyretics), and most people were still protected from severe disease and all were protected against hospitalization.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Mar. 1, 2021; reviewed: December 23, 2022

Many people will get the vaccine and not experience side effects. This does not mean that the vaccine did not work for them. In the clinical trials side effects occurred at varying rates, for example only about 1 to 20 of every 100 people who received the mRNA vaccine had a fever, but we know that the mRNA vaccine worked for more than 90 of every 100 people.

Last updated: Mar. 1, 2021; reviewed: December 23, 2022

• Most negative effects occur within 6 weeks of receiving a vaccine, which is why the FDA asked the companies to provide 8 weeks of safety data after the last dose.
• mRNA vaccines: The mRNA in the vaccine breaks down quickly because our cells need a way to stop mRNA from making too many proteins or too much of a single protein. But, even if for some reason our cells did not breakdown the vaccine mRNA, the mRNA stops making the protein within about a week, regardless of the body’s immune response to the protein.
  
  Read more about COVID-19 mRNA vaccines in this Parents PACK article, “Long-term Side Effects of COVID-19 Vaccine? What We Know.”
  
  Watch a short video of Dr. Paul Offit explaining why COVID-19 vaccines would not be expected to cause long-term side effects.
   
• Adenovirus-based vaccines: Although the DNA from adenovirus-based vaccines does not break down as quickly as mRNA, the DNA cannot alter our DNA because a gene for the enzyme, integrase, is not present. These vaccines are processed within about 4 weeks, so they would not be expected to cause any long-term effects either.
• Protein-based vaccines: The protein is processed within a few days. 

Last updated: July 21, 2022; reviewed: December 23, 2022

If your daily dose of aspirin was prescribed by your physician following a stroke or heart attack, we recommend speaking to that doctor about whether to stop taking your medication for a day or two prior to vaccination. If, however, your daily dose of aspirin is because you have risk factors for a stroke or heart attack (such as high blood pressure or high levels of “bad” cholesterol) but have never had a stroke or heart attack, you should talk to your doctor about discontinuing the aspirin not only prior to your COVID-19 vaccine, but all together. The data show that while daily aspirin helps prevent second strokes or heart attacks, it does not help prevent first occurrences, even in people who are at increased risk. Our director, Dr. Paul Offit, carefully reviewed the data related to this topic for his book, Overkill: When Modern Medicine Goes Too Far.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Jan. 24, 2022; reviewed: December 30, 2022

While your initial immune response may have been lower, you will likely still have developed some immunity. Even if your immune response is somewhat lower overall, you are likely to develop sufficient levels of immunity to reduce your chance for infection. In addition, even if you were infected, you would be likely to experience disease that is less severe and of shorter duration.

Last updated: Mar. 1, 2021; reviewed: December 30, 2022

In the United States, the ingredients in the vial for dose 1 and dose 2 of the same brand are exactly the same. When people talk about "dose 1" doses and "dose 2" doses, they are just talking about vaccine supply. If you arrive for dose 1 and the person behind you is getting dose 2, they can come out of the same vial.

One vaccine used in some other countries, Sputnik V, has different components in dose 1 and dose 2. Both are adenovirus vector vaccines, but dose 1 uses adenovirus 26 (Ad26) and dose 2 uses adenovirus 5 (Ad5).

Booster doses in the U.S. (Pfizer and Moderna mRNA vaccines) are different from the first two doses because they are bivalent versions, meaning they contain mRNA for the spike proteins of both the original (ancestral) strain and, currently, the BA.4/BA.5 Omicron variants.

Last updated: December 30, 2022

Yes. It is okay to get the second dose (or the booster dose) in the other arm as the immunity generated by the first dose will be circulating in your body watching for a potential exposure.

Indeed, individuals who experience a delayed reaction at the injection site (a rash that develops a few days to a couple of weeks after receipt of the vaccine) are recommended to get the second dose in the opposite arm.

Last updated: Mar. 1, 2021; reviewed: December 30, 2022

The CDC recommends that people get the same version for all primary doses (first two doses of Pfizer, Moderna or Novavax and first dose of J&J/Janssen) whenever possible.

People getting an additional dose of mRNA vaccine for an immune-compromising condition should also seek the same version as originally received when possible.

Booster doses should be mRNA version vaccines, but they can be from the same or a different manufacturer. 

Last updated: July 21, 2022; reviewed: December 30, 2022

In most cases, individuals do not need to delay receipt of COVID-19 vaccine and other vaccines; however, if given during the same appointment, the vaccines should be administered in different locations (different arms or separated by at least one inch on the same arm).

The one exception is that people who get an orthopox vaccine (mpox/smallpox), particularly teen and young adult males, should consider waiting for 4 weeks before getting a COVID-19 mRNA or protein-based vaccine due to known or potential risks of myocarditis related to individual orthopox and COVID-19 mRNA and protein-based vaccines. However, if the individual is at risk for mpox due to an outbreak or exposure and recently had a COVID-19 mRNA or protein-based vaccine, they should not delay their orthopox vaccination given that they would be trading a real risk for a theoretical risk by delaying.

Watch this short video in which Dr. Hank Bernstein explains the benefits of receiving routine vaccines at the same time as the COVID-19 vaccine.

Last updated: December 30, 2022

If a parent tests positive, they should try to isolate from other members of the household. Others in the home, including any children, should follow guidance based on their immunization status.

See the latest CDC guidance:

• Isolation and Precautions for People with COVID-19
• Isolation graphic
• What to Do If You Were Exposed to COVID-19
• COVID-19 Quarantine vs. Isolation

Last updated: December 30, 2022

Multisystem inflammatory syndrome can occur in children (MIS-C) or adults (MIS-A). Development of symptoms typically occurs about 4 to 6 weeks after SARS-CoV-2 infection and can occur even in those who did not experience symptoms of COVID-19. Often multiple organs and body systems are involved, including effects on the gastrointestinal tract, heart, kidneys, skin, lungs, and eyes. Individuals with unexplained rash, vomiting or diarrhea, shortness of breath or chest pain or palpitations should seek medical care. Some people with MIS-C or MIS-A will require admission to intensive care and a small number may require mechanical ventilation.

Find out more about MIS-C and long COVID-19 in this video with one of CHOP’s infectious diseases pediatricians.

Watch this short video in which Dr. Offit discusses whether MIS-C after COVID-19 infection is going away.

Last updated: December 30, 2022

Long COVID, also known as post-COVID conditions or long-term COVID, is characterized by long-lasting symptoms related to previous SARS-CoV-2 infection. Symptoms can last for weeks or months after viral clearance and resolution of the initial infection. Examples of the types of symptoms that affected individuals report include fatigue, difficulty thinking or concentrating (“brain fog”), headache, change in or loss of taste or smell, dizziness, heart palpitations, chest pain, shortness of breath, cough, joint or muscle pain, anxiety, depression, sleep problems, feelings like “pins and needles,” diarrhea or stomach pain, rash, changes in menstrual cycle, or fever. Symptoms sometimes appear or worsen after physical or mental activity. People, particularly those who experienced severe COVID-19 infections, may also develop new chronic conditions, such as diabetes, heart conditions or neurological conditions.

Scientists continue to research long COVID. Currently, three theories about the causes have been put forth, one or more of which are likely causing the prolonged symptoms:

• Long-term viral replication
• Hyperactive immune responses in response to the virus
• Blood clots (specifically microclots) caused by infection in an array of body organs

Watch this short video in which Dr. Offit discusses what we are learning about long COVID and how these three possibilities would be resolved by different approaches to treatment.

Last updated: December 30, 2022

Vaccinated people do not shed virus as a result of vaccination. Neither the COVID-19 mRNA nor the adenovirus vaccines are composed of live viruses, so there is no infectious virus to spread from a vaccinated person to someone else. The same is true of the newer vaccine, Novavax, which is composed of the single spike protein.

But a vaccinated person can still be infected and potentially spread the virus to others. If they do not have symptoms, they may spread the virus without even knowing they are infected. While vaccinated individuals who become infected can be a source of viral spread, they do not appear to spread as much virus as unvaccinated individuals who become infected because their immune response is able to respond to the infection more quickly – shortening the length of infection and, therefore, the amount of virus produced.

Read more, “Vaccinated or Unvaccinated: What You Should Know.”

Last updated: June 22, 2022; reviewed: December 30, 2022

The mRNA vaccines include:

• mRNA – This mRNA is for the spike protein of SARS-CoV-2, the virus that causes COVID-19.
• Lipids - These are molecules that are not able to dissolve in water. They protect the mRNA, so that it does not break down before it gets into our cells. These can be thought of as little “bubbles of fat,” which surround the mRNA like a protective wall. There are four different lipids in the Pfizer vaccine and three in the Moderna vaccine. One of the lipids in both vaccines is cholesterol. The lipids are the most likely components of the vaccine to cause allergic reactions.
• Salts and amines - The Pfizer vaccine contains four salts. One is table salt. The salts are used to keep the pH of the vaccine similar to that found in the body, so that the vaccine does not damage cells when it is administered. The Moderna vaccine also contains four chemicals to balance the pH, but two are in a class of organic compounds known as “amines” and two are acetic acid and its salt form, sodium acetate. Acetic acid is the main component of vinegar (other than water).
• Sugar – This ingredient is literally the same as that which you put in your coffee or on your cereal. It is used in both of the vaccines to help keep the “bubbles of fat” from sticking to each other or to the sides of the vaccine vial.

These are the only ingredients in the mRNA vaccines.

NOT in the COVID-19 mRNA vaccines:

• Animal Products
• Antibiotics
• Blood products
• DNA
• Egg Proteins
• Fetal material
• Gluten
• Microchips
• Pork products
• Preservatives, like thimerosal
• Soy

Watch this short video in which Dr. Paul Offit talks about the ingredients of COVID-19 mRNA vaccines.

Last updated Feb. 19, 2021; reviewed: December 30, 2022

The adenovirus vaccine includes:

• Adenovirus type 26 (Ad26) containing SARS-CoV-2 spike protein gene and altered so that it cannot replicate
• Stabilizers – Salts, alcohols, polysorbate 80, and hydrochloric acid
• Manufacturing by-products – amino acids

Because the adenovirus-based COVID-19 vaccine is grown in fetal cells. Although the product is highly purified, remnants of these cells may remain in the final product. 

NOT in the COVID-19 adenovirus vaccines:

• Animal Products
• Antibiotics
• Blood products
• Egg Proteins
• Gluten
• Microchips
• Pork products
• Preservatives, like thimerosal
• Soy

Last updated: Jan. 24, 2022; reviewed: December 30, 2022

The protein-based vaccine includes:

• SARS-CoV-2 spike protein
• An adjuvant derived from the soap bark tree (Quillaja saponaria), called Matrix-M
• Stabilizers – Salts (including table salt), polysorbate 80, and hydrochloric acid

NOT in the COVID-19 protein-based vaccine:

• Animal Products
• Antibiotics
• Blood products
• DNA
• Egg Proteins
• Fetal material
• Gluten
• Microchips
• Pork products
• Preservatives, like thimerosal
• Soy

Last updated: June 22, 2022; reviewed: December 30, 2022

No. Neither the mRNA vaccines (Pfizer and Moderna), adenovirus vaccine (Johnson & Johnson/Janssen), nor the protein-based COVID-19 vaccine (Novavax) contain antibiotics.

Watch this short video in which Dr. Hank Bernstein discusses which ingredients are and are not in the COVID-19 mRNA vaccines.

Last updated: June 22, 2022; reviewed: December 30, 2022

Since mRNA is active only in a cell’s cytoplasm and DNA is located in the nucleus, mRNA vaccines do not operate in the same cellular compartment that DNA is located.

Further, mRNA is quite unstable and remains in the cell cytoplasm for only a limited time (See “What stops the body from continuing to produce the COVID-19 spike protein after getting an mRNA vaccine?” below.) mRNA never enters the nucleus where the DNA is located so it can’t alter DNA.

Watch this short video in which Dr. Paul Offit explains why it’s not possible for mRNA vaccines to alter a person’s DNA.

Last updated Dec. 15, 2020; reviewed: December 30, 2022

Adenovirus-based vaccines contain DNA, which enters the nucleus of cells after vaccination, but the virus cannot replicate and the vaccine does not include a necessary enzyme, called integrase. Therefore, the vaccine cannot change a person’s DNA.

Last updated: Mar. 1, 2021; reviewed: December 30, 2022

Both the mRNA and adenovirus vaccines result in production of spike protein that results from mRNA blueprints. Because our cells are continuously producing proteins, they need a way to ensure that too many proteins do not accumulate in the cell. So, generally speaking, mRNA is always broken down fairly quickly. Even if for some reason our cells did not breakdown the vaccine mRNA, the mRNA stops making the protein within about a week, regardless of the body’s immune response to the protein. Once the mRNA is broken down, the blueprint is gone, so the cell can no longer continue to make spike proteins.

Likewise, while the adenovirus-based vaccine delivers DNA and the DNA lasts longer than mRNA, studies have shown that adenovirus-based DNA does not last longer than a few weeks.

Watch this short video in which Dr. Hank Bernstein explains how the mRNA from the COVID-19 vaccine is broken down and removed from the body.

For more details on the process by which spike protein production is limited, see the “mRNA vaccine” section of this article.

Last updated March 28, 2023

This question has two aspects – longevity of the spike protein and effects of current immune responses to future variants. While related, these are not cumulative issues, meaning they involve separate considerations:

• Longevity of the spike protein - The spike protein does not remain in the body for an extended time, nor does it travel around the body. The only thing that remains after the vaccine is processed are antibodies and memory immune cells that will recognize the virus if we are exposed in the future. The antibodies and memory cells will or will not recognize the variant spike protein. If they do, great – we will have some protection. If they don’t, it will be just like an antibody to some unrelated pathogen (like flu or measles), it will have no effect.
• Effects of current immune responses to future variants - As the virus evolves, it changes, so we might find ourselves dealing with different versions of the virus in the future. Current variants have not changed significantly enough that antibodies produced by our immunologic memory have stopped being protective against severe disease. However, the variants have changed enough that protection against any infection (i.e., non-severe infection) is lower.

Last updated: December 30, 2022

The CDC recommends waiting until your symptoms go away and you are no longer isolating. However, studies have shown that your immune response to the vaccine will be better if you delay vaccination for about three to four months after the infection, so you may want to delay getting your next dose after an infection.

If you happen to be infected, but don’t know because you have not yet developed symptoms or you have an infection without symptoms, the vaccine is not likely to be harmful. Your body’s immune response would develop against both the virus and the vaccine.

Last updated: December 30, 2022

Alcohol suppresses the immune system, so it would be advisable not to drink alcoholic beverages for about 2 weeks after getting vaccinated.

Last updated: Dec. 31, 2020; reviewed: December 30, 2022

Giving blood after getting the COVID-19 vaccine will not diminish the resulting immune response, which mostly builds in the lymph nodes near the injection site. Likewise, the American Red Cross (ARC) does not require a delay following vaccination with the vaccines currently approved for use in the U.S.; however, individuals must know which brand of vaccine they received and show the immunization card if possible. More details about blood donation are available on the ARC website.

Last updated: Mar. 18, 2021; reviewed: December 30, 2022

The mRNA vaccines (those by Pfizer and Moderna) and the protein-based vaccine (Novavax) do not contain fetal cells.

But, the adenovirus-based vaccines, like Johnson & Johnson/Janssen’s, use cells originally isolated from fetal tissue (often referred to as fetal cells). These fetal cells are used to grow the vaccine virus.

To replicate, a virus needs to take over a cell’s machinery (See this animation); however, the adenoviruses used in these vaccines have been altered, making them unable to complete the replication process. So, to make the vaccine, these altered viruses need to infect cells that have been changed in a way that allows the defective virus to reproduce. The special cells for this process were isolated decades ago from one of two terminated fetuses and later adapted for the adenovirus reproduction process. Neither of these are used to produce any existing vaccines grown in fetal cells:

• HEK-293 — This is a kidney cell line that was isolated from a terminated fetus in 1972.
• PER.C6 — This is a retinal cell line that was isolated from a terminated fetus in 1985.

These two cell lines have been maintained in the laboratory, and no additional fetuses are needed to produce adenovirus-vector vaccines.

In this short video, Dr. Paul Offit addresses fetal cells and COVID-19 vaccines.

You can find out more about the adenovirus-based vaccines and fetal cells in this Vaccine Update article.

Last updated June 22, 2022; reviewed: December 30, 2022

Over time, as we have learned more about the virus that causes COVID-19 (called SARS-CoV-2) and witnessed its ability to change, the recommendations about the number of vaccine doses for individuals have changed. As with any developing area of science, the recommendations and terminology may change more in the future. However, at the moment, here is a summary to help you sort through the recommendations:

Primary series

Primary doses of a vaccine are those doses needed to induce a robust immune response the first time someone is introduced to a pathogen. A person is not considered immunized against a particular pathogen until they have completed all primary doses. Currently in the U.S., primary dose recommendations are slightly different for healthy people and those with a condition that compromises their immune system. Likewise, variations exist across vaccines.

Most individuals

• Pfizer – Two primary doses for those 6 months and older.
• Moderna – Two primary doses for those 6 months and older.
• Novavax – Two primary doses for those 12 years and older.
• J&J/Janssen – One primary dose for those 18 years of age and older, although the U.S. CDC has expressed a preference for individuals to use other types of COVID-19 vaccine when possible due to rare, but real, severe side effects.

People with immune-compromising conditions

• Pfizer – Three primary doses for all age groups.
• Moderna – Three primary doses for all age groups.
• Novavax – Two primary doses for those 12 years and older.
• J&J/Janssen – One primary dose of J&J/Janssen followed 4 weeks later by one dose of a bivalent mRNA-based version (booster). This vaccine is only for those 18 years and older.

Booster doses

Booster doses of a vaccine are those doses needed to “remind” the immune system about a pathogen. Booster doses of COVID-19 vaccines have been demonstrated to increase the level of antibodies circulating in the blood, thereby decreasing the chance for any infection (e.g., mild infection) for a period of a few months after receipt of the dose. Currently, in the U.S., booster dose recommendations are for bivalent mRNA vaccines (Pfizer or Moderna). The Novavax vaccine can also be used in limited situations. Although the CDC recommends booster dosing at least two months after the last dose, longer intervals, such as three to six months later, are preferred.

Most individuals

• Pfizer – One booster dose at least two months after the last primary dose for those 6 months of age and older.
• Moderna – One booster dose at least two months after the last primary dose for those 6 months of age and older.
• Novavax – One booster dose at least two months after the last primary dose for those 12 years of age and older.
• J&J/Janssen – One booster dose at least two months after the primary dose for those 18 years and older.

People with immune-compromising conditions

• Pfizer – One booster dose at least two months after the third dose for those 6 months of age and older with immune-compromising conditions.
• Moderna – One booster dose for those 6 months of age and older with immune-compromising conditions for a total of four recommended doses for this group. Booster doses should be given at intervals of at least two months after receipt of the third dose.
• Novavax – One booster dose for those 12 years of age and older at least two months after receipt of the second dose.
• J&J/Janssen – One booster dose at least two months after the primary dose for those 18 years and older.

Last updated: December 30, 2022

We are still learning how long immunity lasts after infection or vaccination. The latest information shows that:

• Following infection people are not likely to be re-infected within 90 days of infection. However, more work is needed to understand how long immunity lasts following infection, particularly because the virus that causes COVID-19 continues to change. Immunity against mild disease doesn’t last as long as protection against severe disease.
• Thus far following vaccination, most people are protected against severe disease regardless of variant. Protection against mild illness caused by omicron and omicron variants is less robust. As such, additional booster doses can benefit certain high-risk groups, including those 75 years and older, those with immune-compromising conditions that put them at higher risk of severe infection, those receiving immune suppressive treatments, and those with medical conditions that put them at higher risk of serious COVID, such as chronic lung, kidney or heart disease.

Last updated: December 30, 2022

People who had COVID-19 are recommended to get the vaccine about 3 to 4 months after they have recovered. Some studies have indicated two benefits:

• Vaccination more consistently produces protective immune responses than infection.
• Vaccination provides a wider breadth of protection based on the types of memory responses produced.

In addition, studies have suggested that infection followed by vaccination provides better protection than either vaccination or infection alone.

Last updated: December 30, 2022

Currently, the Centers for Disease Control and Prevention (CDC) recommends getting all primary doses of the same mRNA or protein-based vaccine unless the supply does not allow for doing so.

The CDC recommends getting mRNA or protein-based vaccines when possible unless an individual requires or prefers J&J/Janssen for a particular reason or if the supply limits one’s choice.

Booster doses are recommended to be mRNA-based versions. The protein-based vaccine (Novavax) is approved for booster doses in limited situations.

Last updated: December 30, 2022

SARS-CoV-2 infections can be a minor hindrance or lead to severe disease or even death. Likewise, some people, including children, experience lingering symptoms, called “long COVID,” which is yet to be understood. While hygiene measures such as social distancing, handwashing, and wearing masks offer some help, the best way to stop this virus is to generate SARS-CoV-2-specific immunity. No virus has ever eliminated itself by inducing natural immunity in a large percentage of the population. Only herd immunity induced by vaccination can eliminate viruses, as has now been shown for smallpox and two of the three different types of poliovirus.

For more information, watch this short video of Dr. Paul Offit addressing “Why does it matter if I don’t get the COVID-19 vaccine?”

Last updated: June 23, 2022; reviewed: December 30, 2022

The Pfizer mRNA vaccine requires two or three doses for those greater than 6 months of age, depending on age and immune status. Protection against severe disease is greatest about 2 weeks after the last primary dose.

The Moderna mRNA vaccine requires two or three doses for the primary series depending on immune status.

The protein-based vaccine (Novavax) requires two doses. Immunity will be most robust about two weeks after the second dose.

The adenovirus vaccine (Johnson & Johnson/Janssen) requires one dose. While people will have some immunity about two weeks after being vaccinated, protection will be more robust about one month after receipt of the vaccine. Likewise, individuals are recommended to get a second dose at least 8 weeks after the first dose to further bolster their immunity. The second dose is preferred to be an mRNA-based version.

Last updated: December 30, 2022

Current variants circulating in the U.S. are being monitored for their ability to spread, cause more severe disease, and evade vaccines and treatments. Current vaccines will protect most against severe disease and death; however, booster doses have been recommended to better protect against less severe disease for at least a few months after receipt. Because the original mRNA vaccines remain effective against severe disease and death for most people, vaccination of unvaccinated individuals is still recommended.

Last updated: December 30, 2022

Pregnant women were not included in the early COVID-19 vaccine studies, but some participants were either pregnant and did not know it or became pregnant during the course of the study. Likewise, tens of thousands of pregnant women have been safely immunized since the COVID-19 mRNA vaccines became available.

With data from thousands of these women now in hand, no concerns have been identified and the vaccine works. Further, we now know that:

• Pregnant women are at higher risk for severe COVID-19 compared with those of the same age who are not pregnant.
• Vaccination during pregnancy also affords some protection to the baby in the months after delivery and before they are old enough to be vaccinated.

Pregnant women who get the COVID-19 vaccine should take acetaminophen if they develop a fever after vaccination, as fever during pregnancy can negatively affect a developing baby. Taking acetaminophen during pregnancy has been found to be safe.

In this short video, Dr. Hank Bernstein discusses COVID-19 vaccination during pregnancy.

You can read more about pregnancy and COVID-19 vaccines in this Vaccine Update article.

Drs. Paul Offit and Ripudaman Minhas discuss vaccines, pregnancy, development and autism in this video.

Last updated: December 30, 2022

Yes. Although women who are breastfeeding were not included in the clinical trials, data have indicated that COVID-19 is not transmitted through breast milk, so it is not expected that vaccination would cause a concern either.

In addition, women do not need to delay breastfeeding for any period of time before or after they have been vaccinated.

Babies may benefit from antibodies or immune cells introduced through breast milk after the mother is vaccinated. This is called passive immunity.

Both the Academy of Breastfeeding Medicine and the American College of Obstetricians and Gynecologists support this approach.

In this short video, Dr. Hank Bernstein discusses COVID-19 vaccination when breastfeeding.

You can read more about breastfeeding and COVID-19 vaccines in this Vaccine Update article.

Drs. Paul Offit and Amna Husain discuss vaccines and breastfeeding in this video.

Last updated: December 30, 2022

Yes, women who are trying to get pregnant can get the COVID-19 vaccine. Likewise, if a woman finds out she is pregnant after starting vaccination but before receiving all recommended doses, she can still continue with vaccination, and it is important to do so since pregnant women are at higher risk of being hospitalized and having pre-term births if infected with COVID-19 during pregnancy.

Last updated: June 23, 2022; reviewed: December 30, 2022

No, you do not need to delay pregnancy. The COVID-19 vaccines do not present a cause for concern related to pregnancy.

Last updated: Jan. 25, 2022; reviewed: December 30, 2022

Some people experience swelling of the lymph nodes under their vaccinated arm after getting the COVID-19 mRNA vaccine. Because this could be mistakenly identified as spread of breast cancer to lymph nodes, delaying the mammogram can prevent the chance of this happening.

Last updated: Jan. 25, 2022; reviewed: December 30, 2022

People occasionally experience swelling of the lymph nodes near the vaccine injection site, which could interfere with interpreting the results of the MRI depending on what location is being imaged.

Last updated: Jan. 25, 2022; reviewed: December 30, 2022

Generally speaking, it would be recommended to wait about a week after getting the mRNA vaccine and a few weeks after getting the adenovirus-based vaccine before getting bloodwork. Delays are not likely to be needed after receipt of the protein-based vaccine. However, it would be better to inquire with the healthcare provider who ordered the bloodwork as they have the benefit of knowing the reason for the bloodwork, the type of tests ordered, and the patient’s medical history. As such, they will be in the best position to offer this guidance for each individual situation.

Last updated: July 21, 2022; reviewed: December 30, 2022

People with immune-compromising conditions may get the COVID-19 vaccine as long as they are not in one of the following categories:

• Severe allergy to a vaccine component (i.e., one that causes anaphylaxis or requires medical intervention)
• History of severe allergy to any vaccine or injectable medication

However, it is recommended that individuals with compromised immune systems discuss their personal risks and benefits with a healthcare provider to determine whether to receive the vaccine or if they may need additional doses.

Knowing the potential for a lower immune response, if someone with an immune-compromising condition decides to get vaccinated, it will be important to get all recommended doses, depending on their condition (See “How many doses of a COVID-19 vaccine will be needed?” for more details). They may also choose to continue practicing other public health measures during periods of high virus circulation.

Last updated: June 23, 2022; reviewed: December 30, 2022

People with a history of Guillain-Barré Syndrome (GBS) can get the COVID-19 vaccine, as long as they do not have another condition that puts them among the people recommended against vaccination. Since a small number of cases of GBS have been identified following receipt of the adenovirus-based COVID-19 vaccine (J&J/Janssen), a preference has been expressed for use of the mRNA- or protein-based vaccines. GBS cases following receipt of the adenovirus-based vaccine have been rare (about 1 of 100,000 people), but if an individual developed GBS within 6 weeks of receiving it, they should only get future doses of a different type of vaccine. Individuals who had GBS in the past, have a “precaution” against the adenovirus-based COVID-19 vaccine, meaning they may be able to get it if they and their healthcare provider determine that the benefits outweigh the potential risks; however, since mRNA and protein-based vaccines are more widely available, preferred, and have not been linked to GBS, most individuals and their healthcare providers will likely settle on one of these versions for vaccination.

A note about GBS and influenza vaccines

Some people wonder if they can get the COVID-19 vaccine if they developed GBS following receipt of an influenza vaccine. Since COVID-19 and influenza (flu) vaccines are made differently, people with this history would not be expected to have an issue with COVID-19 vaccine. As such, they are still recommended to get COVID-19 vaccine.

Finally, many people are incorrectly told that if they had GBS, they cannot get a flu vaccine. However, most people with a history of GBS can get the flu vaccine. Only people who were diagnosed with GBS less than 6 weeks after receipt of influenza vaccine are considered to have a “precaution” for receipt of influenza vaccine, meaning that the patient and the healthcare provider should discuss the relative risks and benefits associated with getting the influenza vaccine. In fact, studies have shown that influenza disease presents a greater risk of GBS than influenza vaccination. Find out more:

• ”Vaccines and Guillain-Barré Syndrome” webpage
• Guillain-Barré Syndrome (GBS) & Vaccines: The Risks and Recommendations, September 14, 2021, Parents PACK newsletter

Last updated: July 21, 2022; reviewed: December 30, 2022

People with mild cold-like symptoms are not prevented from getting the vaccine. However, if they are not feeling well, their symptoms just started, or their symptoms are getting worse, they may want to delay vaccination until they feel better; otherwise, they might not be able to tell effects of illness from those of the vaccine. If they are uncertain, they should speak to their doctor, who has the benefit of their medical history and will be in the best position to help them weigh the potential pros and cons.

Last updated: Mar. 1, 2021; reviewed: December 30, 2022

The receipt of dermal fillers does not prevent someone from getting the COVID-19 vaccine. While a few people with dermal fillers have experienced swelling in the area of the fillers following receipt of the mRNA vaccine (most often, but not exclusively, Moderna), these events have been extremely rare and have responded to treatment. Likewise, at least one case has also been identified following COVID-19 infection.

You can read more from the American Society of Plastic Surgeons and the American Society for Dermatologic Surgery.

Last updated: December 30, 2022

Patients on blood thinners can get the COVID-19 vaccine. However, because the vaccine is given intramuscularly, the risk for bleeding is slightly greater for these individuals. As such, they should tell the healthcare provider administering the vaccine about their use of an anticoagulant. The vaccine itself does not increase the risk for this group of patients.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Sept 28, 2021; reviewed: December 30, 2022

As long as you are not still sick from your recent infection, you can get the COVID-19 vaccine even if you are taking an antibiotic. But, if you are still having symptoms, you should wait until you are feeling better, so that it is easier to tell if any new symptoms are from your infection or the vaccination.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Sept. 28, 2021; reviewed: December 30, 2022

You do not need to stop taking antiviral medication before vaccination. Because the COVID-19 vaccines being used in the U.S. do not rely on viral replication, antivirals should not affect development of the immune response. However, if you are still experiencing symptoms of the infection for which the antivirals were prescribed, you should wait until you are feeling better before getting the vaccine. This will allow you to distinguish symptoms from your infection with side effects from the vaccine.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: June 23, 2022; reviewed: December 30, 2022

Taking biologics, like Humira, is not a reason to forgo COVID-19 vaccination as per CDC guidelines. However, patients taking these types of medication may wish to consult with their doctor to discuss the potential risks and benefits of getting the COVID-19 vaccine, given that these types of medications are often prescribed for individuals with immune-compromising conditions. As a result, there may be other considerations related to the potential risks and benefits of vaccination.

For general information about vaccines and biologics, check out this printable Q&A sheet.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Jan. 25, 2021; reviewed: December 30, 2022

People can have dental procedures after receipt of the COVID-19 vaccine. Vaccine-induced immunity should not be affected by nitrous oxide or antibiotics that might be prescribed after the procedure.

Last updated: Jan. 25, 2021; reviewed: December 30, 2022

You should speak with your doctor to determine whether the quantity of steroids that you are receiving is suppressing your immune system. If so, you should hold off on receiving vaccines until the effect of the steroids has worn off.

Find out more in this Parents PACK article, "Medications and COVID-19 Vaccines: What You Should Know."

Last updated: Jan. 25, 2021; reviewed: December 30, 2022

It would not be expected that the COVID-19 vaccines would cross the blood-brain barrier (BBB) for a few reasons.

mRNA vaccines:

• Most of the protein that is made is bound to cells - The vaccine is injected into muscle, where dendritic cells in the area use the mRNA to make the COVID-19 spike protein. These dendritic cells, after making the spike protein, put the protein (not the mRNA) on the cell surface, travel to the nearest lymph node, and stimulate other cells of the immune system to make an immune response against the protein. This process is typical of our adaptive immune system, which you can find out more about in this animation, or you can watch this animation that describes how the mRNA vaccine is processed.
• Even if the protein left the cell whole (which it doesn’t), it is too large to cross the BBB.

Adenovirus vaccine:

• As with the mRNA vaccine, pieces of the protein that results from vaccine processing are put on dendritic cells that travel to the nearest lymph node. See how the adenovirus-based vaccine is processed in this animation.
• The virus used to deliver the vaccine is also too large to cross the BBB.

Protein-based vaccines (e.g., Novavax) would not be expected to cross the BBB either as the proteins are too large.

Last updated: June 23, 2022; reviewed: December 30, 2022

Antibody-dependent enhancement (ADE) occurs when the antibodies from a previous infection (or vaccination) help the virus gain access to cells rather than blocking access to cells. Getting an infection after vaccination does NOT provide evidence of ADE. These are two distinct immunologic phenomena.

ADE has not been identified as a concern related to SARS-CoV-2 infection or following COVID-19 vaccination. In fact, a body of evidence has suggested that ADE is not a concern:

• First, most people have been infected with other coronaviruses in their lifetime, and ADE has not been identified as a result of these infections.
• Second, in human studies, people previously infected with coronavirus were infected with different types of coronavirus, and they did not experience enhanced disease.
• Third, experimental animals vaccinated against SARS-CoV-2 did not develop enhanced disease when challenged, or infected, with the virus.
• Fourth, when people with COVID-19 received plasma containing SARS-CoV-2 antibodies, they did not experience enhanced disease.
• Finally, millions of people have been vaccinated against COVID-19, and some of them have subsequently been infected with SARS-CoV-2, or one of its variants, and none of them have shown evidence of ADE.

Scientists will continue to monitor the SARS-CoV-2 variants; however, it would not be expected that ADE will become an issue with the rise of new variants, as it has not been with the many types of coronaviruses and the existing SARS-CoV-2 variants to date.

Watch a short video in which Dr. Paul Offit explains why COVID-19 vaccines are unlikely to cause ADE.

Last updated: Jan. 25, 2022; reviewed: December 30, 2022

Infertility has not been found to be an issue in women or men infected with or vaccinated against COVID-19.

Unfortunately, misinformation about fertility-related issues has been circulating online. These concerns take a few forms:

1. Compromised fertility in the vaccine recipient – Original concerns related to a placental protein, called syncytin-1. This protein is associated with the placenta during pregnancy. Online claims promoted a paper suggesting that a small number of similar amino acids in the spike protein and the placental protein would cause vaccine-induced antibodies to react against syncytin-1. Since human proteins are made using the same 20 amino acid building blocks, many proteins have short sections that are similar to one another. However, most of our antibodies do not cross-react with other proteins because a variety of other factors come into play. The most important of which is antibody specificity to the three-dimensional version of its target. As such, while a theoretical paper like the one previously mentioned can generate an interesting hypothesis, the idea requires clinical confirmation, which never materialized for this idea that unfortunately spread quickly and, quite frankly, unnecessarily scared people.
2. Some concerns related to males, and whether the vaccines could decrease sperm count. While fever can cause a temporary decrease in sperm count, there is no biologically plausible reason to expect that the vaccines would cause any long-term effect on sperm count.
3. Compromised fertility in individuals near someone who recently received COVID-19 vaccine – This misperception conflates two concepts: effects on fertility and viral shedding. As mentioned above, the vaccines do not affect fertility in the vaccinated person, so there would not be a reason to expect that they would affect someone else’s fertility. Second, it assumes that recently vaccinated individuals shed virus or spike protein. Neither of these occur. While these vaccines cause the body to generate spike protein, they do not cause production of whole virus particles, nor do parts of the vaccine migrate to the nasal cavity. As such, a recently vaccinated person does not shed any part of the virus and cannot, therefore, spread vaccine-related components to another person.

Watch this short video in which Dr. Paul Offit discusses COVID-19, the vaccines and infertility.

You can read more about fertility and COVID-19 vaccines in this Vaccine Update article.

This Parents PACK article about vaccination of children 5 to 11 years of age also addresses fertility-related concerns.

Last updated: Jan. 25, 2022; reviewed: December 30, 2022

The CDC recommendations allow people to get COVID-19 vaccines at the same time as most other vaccines. This decision was made due to experience with more than one vaccine given close in time with COVID-19 vaccines during emergency situations and with more understanding of the effects of the COVID-19 vaccines, suggesting a low likelihood of interference. However, studies will continue to monitor responses during these situations to ensure that unexpected events do not occur. The CDC change also considered the increased susceptibility of individuals who missed routinely recommended vaccines during the pandemic. If an individual is uncomfortable getting both vaccines at once and can conveniently return for a second visit, the vaccines can be separated by two weeks, but if the individual can’t return in a timely manner, it is acceptable to give both vaccines at the same visit but in separate locations.

One notable exception is a recommendation to delay COVID-19 mRNA- or protein-based vaccination (Pfizer, Moderna, or Novavax) for 4 weeks after receiving an orthopox vaccine (mpox/smallpox), due to risks of myocarditis associated with both types (COVID-19 and orthopox) of vaccine. This is particularly relevant for teen and young adult males. However, if the individual was recently vaccinated against COVID-19 and then is recommended to get the orthopox vaccine due to an outbreak or known exposure, they should not delay vaccination.

Watch this short video in which Dr. Hank Bernstein explains the benefits of receiving routine vaccines at the same time as the COVID-19 vaccine.

Last updated: July 21, 2022; reviewed: December 30, 2022

The lingering effects of COVID-19 are concerning, and we still have much to learn about them. A vaccine will help from the point of view that if it decreases infections, fewer people will experience illness and, therefore, fewer people will experience long-term effects. But, it is not likely that a vaccine will address these effects in someone who was already infected. Find out more about progress understanding the lingering effects in the response to “What is long COVID?”

Last updated: December 30, 2022

The COVID-19 vaccines available in the U.S. do not contain any blood products, including red blood cells, white blood cells or platelets. 

Watch this short video in which Dr. Offit talks about the ingredients used in the COVID-19 mRNA vaccines.

Last updated: Mar. 1, 2021; reviewed: December 30, 2022

During the height of the COVID-19 pandemic, several tools were developed to help public health officials, governments, businesses, and individuals make informed decisions. These tools typically used county-level data to provide guidance. Some of these tools are no longer in operation; however, you can still assess county-level risk using the “COVID-19 event risk assessment planning tool,” developed by teams at Georgia Institute of Technology and Applied Bioinformatics Laboratory. This tool shows the percent chance that at least one person will be COVID-19 positive in gatherings of different sizes. The calculations are based on results of data from COVID-19 antibody blood tests.

Last updated: June 23, 2022; reviewed: December 30, 2022

The viral genome is not a product; it represents scientific knowledge, so organizations like the FDA or CDC would not have “oversight” over the information. However, this question gets at the heart of how science is done. Scientists by their nature are skeptics, and the scientific process is designed to challenge rather than accept results. In this manner, several points offer reassurance that the genomic sequence was vetted for accuracy:

• The scientists who reported the genome isolated samples from several patients to examine the genome. Said another way, their data were not based on a single person’s infection. They had to confirm for themselves and for the quality of their research that what they found was accurate. They could not assume that the same virus was causing infections without actually gathering evidence of such.
• Once they completed their study, they had to share it with colleagues, who would critically review it and maybe even ask for more experiments or clarifications before they could publish a paper sharing their results with the world. Peer-review is critical to the scientific process, which is why you may have heard about data that were not yet peer-reviewed during the pandemic. For scientists, that means that the work has not yet been vetted.
• Several other labs also isolated samples from patients and reproduced the process. Their papers were also peer reviewed before publication. Reproducibility is a second critical component of the scientific process. Even if the genomic information passed peer review and was published, if other labs did not find the same thing, the information would be called into question.

In this manner, the pillars of scientific integrity — peer review and reproducibility — can offer everyone reassurance that the genomic sequence was accurate— not to mention the fact that vaccines based on the information have been effective at preventing infection.

Last update: Mar. 31, 2021; reviewed: December 30, 2022

While COVID-19 vaccines were developed more quickly than ever before, it was imperative that speed did not decrease safety.

In this case, the timeline was shortened without sacrificing quality by:

• Skipping phase I or combining phase I with phase II trials — Since phase I studies include a small number of people and evaluate whether the candidate vaccine causes an immune response and is safe, scientists could look at data from a group of people as phase II was progressing to make these evaluations.
• Manufacturing “at risk” — While completing the large phase III clinical trials, manufacturers began producing the vaccine, so that if it was shown to be safe and effective, they would have large numbers of doses ready. The reason this is not typically the approach is because if the vaccine does not work, the manufacturer will have spent a significant amount of money to produce something that needs to be thrown away.
• Support efforts — While waiting for a vaccine to be ready, many other aspects of vaccine delivery were prepared, including:
  • Developing plans for how to distribute the first, limited quantities available
  • Ensuring adequate supplies for distributing and administering vaccine, like vaccine vials, syringes and other equipment needed to vaccinate
  • Establishing mechanisms for distribution to large subsets of the population, especially in countries in which mechanisms may not currently be in place. For example, many countries do not have standard programs for vaccinating older adults. So, planning how to reach those people, without unintentionally exposing them to a crowd in which the virus may be spread, was something that could be planned during vaccine development.

Last updated: Dec. 15, 2020; reviewed: December 30, 2022

COVID-19 vaccines do not contain microchips. This idea is based on a false narrative and misinformation campaign waged online. You can find out more about where this idea came from on snopes.com.

Last updated: Dec. 15, 2020; reviewed: December 30, 2022

A baby’s vaccines should not be anticipated to protect the baby from COVID-19. So, when trying to decide when it may be safe for family to meet the baby during COVID, parents should not rely on other vaccinations as a source of protection. While some have hypothesized that other vaccines may be protective, this protection would not be specific to COVID-19.

If the baby is at least 6 months of age, she can receive the COVID-19 vaccine; however, she should not be considered immune until at least 2 weeks after receipt of her last dose.

Last updated: Dec. 15, 2020; reviewed: December 30, 2022

This section of the page will house video resources and interviews related to COVID-19.

Vaccine Makers Project videos and animations
The Vaccine Makers Project (VMP) is the classroom program of the Vaccine Education Center (VEC). VMP resources include a variety of science-based animations that show not only how COVID-19 vaccines work, but also how viruses take over our cells and how our immune systems work.

Talking about Vaccines with Dr. Paul Offit: COVID-19
This VEC playlist features several short videos in which Dr. Offit addresses common questions about COVID-19.

Talking about Vaccines with Dr. Hank Bernstein: COVID-19
This playlist features a series of short videos in which Dr. Hank Bernstein answers common questions about COVID-19 vaccines.

My COVID-19 Vaccine Experience
These short videos share personal experiences and decision-making related to receipt of the COVID-19 vaccine.

Perspectives on COVID-19 Vaccine for Kids
These short videos feature personal experiences from clinicians caring for kids with COVID-19; families affected by flu, another virus sometimes perceived as insignificant in kids; and survivors of polio, another virus that causes long-term effects.

Vaccinate Your Family’s COVID-19 Vaccine Updates: Zoom Series Featuring Top Officials from FDA and CDC, series hosted by Vaccinate Your Family

Last updated: December 30, 2022