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 mid-September 2023, more than 2,300 children 17 years of age or younger have died from COVID-19. While this is a small number compared with the more than 1.1 million deaths in the U.S., for those 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.

For a more detailed look at the considerations related to COVID-19 vaccination of children and a series of resources, check out the March 2023 issue of Parents PACK and this April 2023 article penned by Dr. Offit, the VEC’s director.

You can also check out these videos of clinicians, polio survivors, and families affected by influenza, another disease often thought to be “not too bad.”

Last updated: September 20, 2023

The U.S. has three approved COVID-19 vaccines as of the fall of 2023; however, the two that are most often used based on the current vaccine recommendations are those described as “mRNA vaccines.” Find out more about each vaccine:

1. Pfizer mRNA bivalent vaccine – This vaccine contains mRNA to protect against the spike protein from the XBB.1.5 variant of SARS-CoV-2, the virus that causes COVID-19. The bivalent Pfizer vaccine that contained two spike proteins is no longer authorized for use in the U.S. Available in three different doses based on the age of the individual (3 micrograms per dose for 6 months to 4 years, 10 micrograms per dose for 5 to 11 years, and 30 micrograms per dose for 12 years and older).
2. Moderna mRNA bivalent vaccine – This vaccine also contains mRNA to protect against the spike protein from the XBB.1.5 variant of SARS-CoV-2. Likewise, the bivalent Moderna vaccine is no longer authorized for use in the U.S. Two doses are available and are based on the age of the individual (25 micrograms for 6 months to 11 years and 50 micrograms for 12 years and older).
3. Novavax protein-based vaccine – This vaccine only contains the spike protein from the ancestral strain of SARS-CoV-2. While the company has made a version that contains the spike protein against the XBB.1.5 variant, as of late September, it has not been licensed for use by the FDA. As such, it is only recommended for limited use in situations which an individual can’t or is unwilling to get the mRNA version vaccines. This vaccine is approved for those 12 years and older who did not previously get a COVID-19 vaccine. In this situation it should be administered as a two-dose series separated by three to eight weeks. As a booster dose, it can be given as a single dose to those 18 years and older.

A fourth vaccine, J&J/Janssen adenovirus-based vaccine, is no longer available in the U.S., but it is still used in other countries. This vaccine contains a replication-defective adenovirus that has been altered to include the gene (DNA) of the spike protein for the original SARS-CoV-2 virus. Because of the availability of other vaccines and due to the rare but serious side effects associated with this vaccine (i.e., Guillain-Barre syndrome (GBS) and thrombosis with thrombocytopenia syndrome (TTS)), and its monovalent formula, this vaccine was removed from the U.S. vaccine supply in the spring of 2023.

Last updated: September 20, 2023

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.

Because this vaccine contains the spike protein against only one type of COVID-19 (the original strain), it’s recommended use is limited in the U.S.

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; reviewed: September 20, 2023

The general rule of thumb related to CDC guidance is that individuals in the U.S. receive a single booster at least eight weeks after their last dose of COVID-19 vaccine. However, some nuance exists for different age groups and vaccines, as shown on this CDC reference table. As such, if you are not sure whether you need an additional dose, we recommend speaking to your healthcare provider.

Adults 65 years of age and older are recommended to get only a single dose of the updated (2023-2024) vaccine.

Moderately or severely immune compromised individuals may require more doses than their age-matched, immune competent counterparts; however, the recommendations vary based on vaccination history and age, so it is important to talk to your healthcare provider about your or a family member’s specific needs. The recommendations for this group are summarized on this CDC reference table.

Last updated: September 20, 2023

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.

Vaccination and myocarditis

By June 2022, almost 55 million doses of Pfizer’s mRNA vaccine were given to children and teens from 5 to 17 years of age. Of those, 635 cases of myocarditis were diagnosed. Most cases occurred in males, and the side effect was most likely to occur in the first seven days after the second dose, albeit a small number of cases occurred after receipt of the first dose or a booster dose.

When a group of 5- to 17-year-olds who experienced myocarditis after COVID-19 vaccination were followed to see how they did, about half were hospitalized and none died. Most of those who were hospitalized went home within three days. In addition, according to their cardiologists, three months after the event, more than 6 in 10 were fully recovered and an additional 2 in 10 had likely fully recovered, but tests were still outstanding.

COVID-19 disease and myocarditis

A review of the literature, published in August 2022, found that an individual is at least 7 times more likely to experience myocarditis resulting from a COVID-19 infection than from a COVID-19 vaccine.

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

1. Studies have shown that children younger than 5 years of age do not experience myocarditis following receipt of the COVID-19 mRNA vaccines, so vaccinating young children before the risk of myocarditis increases is one way to avoid this potential side effect. Importantly, immunization of our youngest population against COVID-19 has been extremely limited, so it is possible that over time, as more youngsters are vaccinated, we would identify a low risk for myocarditis in young children as well. However, it is also possible that we would see a greater risk from infection compared with vaccination. These are the types of information we need to continue working toward understanding when it comes to this disease.

2. For those at greater risk of this side effect, increasing the time between doses to at least eight weeks, appears to lessen the risk of this side effect.

3. While older individuals can experience myocarditis in the first week after vaccination, the risk is greatest for males between 12 and 39 years of age. The risk for females is lower than for males, but still can occur and is more prevalent between 12 and 29 years of age.

4. 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.

5. 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.

Last updated: September 20, 2023

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.

Last updated: September 20, 2023

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 saw that the levels of memory cells generated by both the mRNA (Pfizer and Moderna) and adenovirus-based (J&J/Janssen) vaccines were 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. In truth, prevention of serious illness is the only reasonable and attainable goal for a virus like SARS-CoV-2, which has a short incubation period.

Watch this video to hear Dr. Offit talk about the Fall 2023 COVID-19 recommendations.

Last updated: September 20, 2023

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 with blood-related cancers, regardless of current treatment status, including those with chronic lymphocytic leukemia, non-Hodgkin lymphoma, multiple myeloma and acute leukemia
• 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 (e.g., B-cell depleting agents)

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: April 25, 2023; reviewed September 20. 2023

Yes. The CDC has indicated that COVID-19 vaccine can be administered at the same visit as any other vaccines (including influenza and RSV vaccines as well as nirsevimab, the RSV monoclonal antibody prevention for infants).

One exception, however,  is for people, particularly young males, who need both a COVID-19 and an orthopoxvirus (mpox or smallpox) vaccine. These people should consider waiting at least four weeks between receipt of the vaccines due to increased risk of myocarditis.

Vaccines given at the same visit should be given in different locations separated by at least one inch.

Last updated: September 20, 2023

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: September 20, 2023

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.

Last updated: July 21, 2022; reviewed: September 20, 2023

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 to 10 per 100,000 vaccine recipients; however, this risk increases in males between 16 and 39 years of age to about 1 per 10,000 vaccine recipients. These numbers are lower in females. They are also lower than if people are infected with the virus that causes COVID-19, which increases the risk of myocarditis at least sevenfold. 
• 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: April 26, 2023; reviewed September 20, 2023

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 11 years of age) is one-half the dose (25 micrograms) of their vaccine for those 12 years and older (50 micrograms for 12 years and older). Pfizer’s vaccine for the youngest children (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 bivalent vaccine dose for 5- to 11-year-olds is one-third the adult dose (10 micrograms).

See more about the importance of vaccinating children against COVID-19:

• March 2023 issue of Parents PACK
• April 2023 article penned by Dr. Offit, the VEC’s director

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: September 20, 2023

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. The one exception is if your child started with the Pfizer vaccine at age 4 and then turns 5, they will still be given the third dose of the vaccine for younger children (3 micrograms).

Last updated: April 26, 2023; reviewed September 20, 2023

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; September 20, 2023

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. In addition, five large, national monitoring systems are being used to monitor for these concerns, and to date, no concerning findings have emerged, including in relation to miscarriage, stillbirth, preterm birth, birth defects, pregnancy or post-delivery complications or outcomes, infant or neonatal outcomes, menstrual irregularities or post-menopausal bleeding.

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: April 26, 2023; reviewed September 20, 2023

Individuals vaccinated in another country are recommended to receive one dose of the 2023-2024 mRNA vaccine if they have not received an updated version approved by the FDA or WHO. The dose should be administered at least eight weeks after the most previous COVID-19 vaccine.

Young children (6 months to 4 years of age and people who are considered immune compromised should check with a healthcare provider, as the recommendations may vary somewhat for them based on U.S. recommendations for these groups.

To see which vaccines are approved by the WHO, check this webpage.

Last updated: September 20, 2023

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; reviewed: September 20, 2023

The mRNA (Moderna and Pfizer) vaccines do not 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. This is also the case for the J&J/Janssen vaccine; however, that version is no longer available in the U.S.

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: September 20, 2023

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 vaccines approved for use in the U.S., the short answer is no. The same is true of adenovirus-based vaccines (like J&J/Janssen) although this type of COVID-19 vaccine is no longer approved for use in the U.S. In both cases (mRNA- and adenovirus-based vaccines), only  the gene for a single protein from the virus that causes COVID-19 – the spike protein – is introduced. 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: September 20, 2023

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 delivered to our muscle cells, which make the protein. The protein is then processed by immune system 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: September 20, 2023

Although COVID-19 adenovirus-based vaccines are no longer used in the U.S., they remain in use in some other countries. These 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 muscle 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. Specialized cells of the immune system, called dendritic cells, put pieces of the newly produced 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: September 20, 2023

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: September 21, 2023

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. The 2023 COVID-19 mRNA vaccines use mRNA for the spike protein from a newer variant of the virus (XBB.1.5), so that the antibodies our immune system produces more closely match the surface protein of the SARS-CoV-2 viruses circulating this fall.

Last updated: September 21, 2023

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 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.

People who cannot get the protein-based vaccine (Novavax), but may be able to get the mRNA (Pfizer or Moderna) 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.

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

• Individuals with a history of 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 or MIS-A should delay vaccination until at least 90 days after diagnosis and they experience a return of normal cardiac function and are considered clinically recovered.
• People who experienced myocarditis or pericarditis within 3 weeks of receipt of COVID-19 vaccine are typically advised not to get additional doses of any COVID-19 vaccine. In some instances, individuals and their healthcare providers may decide to proceed with an additional dose based on the risk-benefit assessment. In this situation, symptoms should have resolved and at least 8 weeks should have passed before any additional doses are administered. Note: This does not apply to people with history of myocarditis or pericarditis unrelated to COVID-19 vaccination (including from COVID-19 infection, prior to COVID-19 vaccination, or more than 3 weeks after COVID-19 vaccination), nor does it apply to people with a history of heart disease.

People who should follow special procedures

• 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 can get vaccinated if they don’t have symptoms.
• People with a current infection should wait at least until symptoms resolve but may have a better immune response to the vaccine if they wait at least 3 months after start of symptoms.

Last updated: September 21, 2023

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: September 21, 2023

Common side effects are caused as part of the immune response to each vaccine.

mRNA vaccines: Older children and adults 

Common side effects are caused as part of the immune response to each vaccine.

• 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 to 10 of every 100,000 mRNA vaccine recipients experience this condition, but it is most likely in adults 39 years and younger and more often occurs in males. This condition tends to occur within 4 days of receipt of the second dose, but it 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 this age group, either in clinical trials or since the vaccines have been in use; 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

Adenovirus-based vaccines are no longer available in the U.S., but they are still used in some other countries.

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: September 21, 2023

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: March 1, 2021; reviewed: September 21, 2023

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: March 1, 2021; reviewed: September 21, 2023

• 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.
   
• Protein-based vaccines: The protein is processed within a few days.

Although no longer available in the U.S., it is worth mentioning that the DNA from adenovirus-based vaccines does not break down as quickly as mRNA. The DNA in the vaccine 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.

Last updated: September 21, 2023

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: September 21, 2023

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: March 1, 2021; reviewed: September 21, 2023

Previously unvaccinated children 6 months to 4 years of age and those 5 years and older who are unvaccinated or partially vaccinated AND moderately or severely immune compromised should get all doses of the same brand, except in certain situations. If you are in this group, talk to your healthcare provider to determine the recommendations for your situation.

Those 5 years and older who have completed their initial vaccine series according to the recommendations for their age and immune status can get any brand. For people who are not immune compromised, they are generally considered to have completed their initial series after at least one previous dose. For those who are severely or moderately immune compromised, they are generally considered to have completed their initial series after at least 3 doses of mRNA vaccine or 1 dose of adenovirus- or protein-based vaccine PLUS 1 or more doses of mRNA vaccine. If you are unsure of whether you can switch brands based on your vaccination history, talk to your healthcare provider to help determine the recommendation for your situation.

Last updated: September 21, 2023

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 need to get both an orthopoxvirus vaccine (mpox/smallpox) and a COVID-19 vaccine, particularly teen and young adult males, should consider waiting for 4 weeks between receipt of the two vaccines 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 or at risk for severe COVID-19, they should not delay their 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: September 21, 2023

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; reviewed: September 21, 2023

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. Current theories about the causes include:

• Long-term SARS-CoV-2 replication or reactivation of other viruses that remain in the body from previous infections
• Changes to the immune system’s ability to self-regulate after infection with the virus
• Blood clots (specifically microclots) caused by infection in an array of body organs
• Damage to mitochondria, which are the energy factories in our cells

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

Last updated: September 21, 2023

Vaccinated people do not shed virus following vaccination. COVID-19 vaccines do not contain live viruses, nor do they cause production of whole viral particles. As such, there is no infectious virus to spread from a vaccinated person to someone else.

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: September 21, 2023

The mRNA vaccines include:

• mRNA – The mRNA is for the spike protein of the XBB.1.5 strain 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: September 21, 2023

Adenovirus-based vaccines are no longer available in the U.S.; however, they are used in other countries.

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 and although the product is highly purified, remnants of the fetal 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: September 21, 2023

The protein-based vaccine includes:

• SARS-CoV-2 spike protein from the original SARS-CoV-2 virus; although this vaccine has been updated to replace the original spike protein mRNA with the XBB.1.5 version, as of late September 2023, the updated version has not been approved for use in the U.S.
• 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: September 21, 2023

No. COVID-19 vaccines do not 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: September 21, 2023

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: September 21, 2023

Although adenovirus-based vaccines are no longer available in the U.S., they are still used in some other countries and some people in the U.S. received them previously, so it is useful to know that they cannot change a person’s DNA. 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: September 21, 2023

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; reviewed: September 20, 2023

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. To date, previous vaccination against COVID-19 has produced immunologic memory that remains effective against newer variants. However, in the fall of 2023, COVID-19 mRNA vaccines were updated to include mRNA for a newer variant, XBB.1.5. While the protein-based vaccine has also been updated, in late September 2023, that version has not yet been approved for use.

Last updated: September 21, 2023

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: March 18, 2021; reviewed: September 21, 2023

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

But the adenovirus-based vaccines (no longer used in the U.S.), 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 September 21, 2023

Discussions related to immunity following COVID-19 vaccination have been fraught with confusion and misinformation. To understand what we have learned from scientific studies, it is important to understand some basics related to immunity in general.

A quick look at immunity

When we are infected with a pathogen, our immune system responds by making B cells and T cells. Some of the B cells are short-lived and have the job of being antibody-producing factories that work to stop the infection. A small number of B cells are long-lived to form one part of our immunologic memory. Likewise, some T cells work to kill virus-infected cells to stop the infection and others serve as army generals, producing chemical signals to control the immune response (ramp it up or down as needed). And, as with B cells, a small number of T cells remain long after the infection to form another part of our immunologic memory.

The long-lasting B and T cells protect us against future infections. If the pathogen is detected, these cells reproduce, so that our immune system overcomes the infection more quickly than it did during the first infection. Whereas first infections can take about a week to control, our immune system is usually at full speed within 3 to 5 days during subsequent infections.

What we have learned about COVID-19 immunity

During a COVID-19 infection, our immune systems produce B cells, antibodies, T cells, chemical signals and immunologic memory — just as in other infections. Antibodies have been found to be important for stopping COVID-19 infections, so in the period of weeks to a few months after an infection or vaccination, lingering antibodies afford protection. But, antibodies are not long lived, so after a few months, we need to rely on immunologic memory. Memory B and T cells have been shown to be important for protection against future infections, particularly certain subsets of T cells. However, as described in the previous paragraph, immunologic memory takes a few days (3 to 5 days) to ramp up. So, during this period, the virus will reproduce in the upper respiratory tract, causing cold-like symptoms (e.g., nasal and sinus congestion, runny nose and coughing) for some and positioning the individual to spread the virus to others (even if they don’t have symptoms). Because of this gap period during which symptoms and transmission can occur, some feel the vaccine doesn’t work. However, the same situation is possible following infection or vaccination, and it will always be the case because of the biology of immunologic memory.

With this said, we have also learned that certain groups of people are at increased risk for severe disease if they are infected. This happens for two reasons — their immune system is not strong enough to overcome the infection by the time their immunologic memory ramps up or one or more components of their immunologic memory is lacking. For these people, help in the form of recently developed antibodies (e.g., vaccination during the part of the year when COVID-19 is circulating) or early treatment with antiviral medications (e.g., Paxlovid™) is most important. The people in these groups include those who are:

• Elderly (65 years and older)
• Moderately or severely immune compromised
• Diagnosed with a chronic disease, particularly heart, kidney, liver or lung disease, diabetes, and obesity, among others (See the CDC’s list here.)
• Pregnant

Last updated: September 21, 2023

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 “hybrid immunity,” that is immunity developed as a result of both infection and vaccination, provides better protection than either vaccination or infection alone.

Last updated: September 21, 2023

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. This virus will continue to circulate in the United States and the world for decades if not longer. 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. At this point, most people have some immunity either from vaccination or from infection. However, two points are worth considering:

1. Most of the individuals being hospitalized for COVID-19 at this point are either individuals who are in a high-risk group or individuals who are unvaccinated.
2. Studies have shown that “hybrid immunity,” which is immunity generated when someone has had both vaccination and an infection, is likely superior to immunity generated from infection alone. As such, even people who were previously infected, can benefit from vaccination.

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: September 21, 2023

The Pfizer mRNA vaccine requires one to three doses for those greater than 6 months old, depending on age. Protection against severe disease is greatest about two weeks after the last recommended dose. For those getting a single dose (ages 5 years and older), the working assumption is that these individuals were likely exposed to the virus in the community, so the dose of vaccine will enhance their immunologic memory. Unfortunately, we cannot be sure that these individuals were equally immune to begin with, so time will tell whether their immunity is as robust as it is for those who had multiple doses.

The Moderna mRNA vaccine requires one or two recommended doses depending on age. The same working assumption as described in the Pfizer vaccine paragraph above relates to single dose recipients here as well (those 5 years of age and older).

Currently, the broadest, longest-lasting immunity is induced by either three doses of an mRNA-containing vaccine or at least two doses plus a natural infection.

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

Last updated: September 21, 2023

Current vaccines will protect most against severe disease and death independent of what variants are circulating; however, neutralizing antibodies that develop shortly after receipt of a dose will fade in a few months, so people can still develop mild illness. For some with immune conditions that affect their response to the vaccine or infection and for those of older age, immune protection is less robust, so they are more likely to benefit from an additional dose. See more details in the answer to “How long will vaccine immunity last?”

Last updated: September 21, 2023

With data from tens of thousands of pregnant 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: April 27, 2023; reviewed: September 21, 2023

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 and vaccination has not caused 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: April 27, 2023; reviewed September 21, 2023

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: September 21, 2023

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: September 21, 2023

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: September 21, 2023

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: September 21, 2023

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: September 21, 2023

Most people with immune-compromising conditions may get the COVID-19 vaccine as long as they do not have a severe allergy to a vaccine component (i.e., one that causes anaphylaxis or requires medical intervention).

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. They may also choose to continue practicing other public health measures during periods of high virus circulation.

Last updated: June 23, 2022; reviewed: September 21, 2023

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. A small number of cases of GBS (about 1 of 100,000 people) have been identified following receipt of the adenovirus-based COVID-19 vaccine (J&J/Janssen); however, this vaccine is no longer used in the U.S.

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: September 21, 2023

People with mild cold-like symptoms are not prevented from getting the COVID-19 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. This advice is similar for other vaccines as well.

Last updated: March 1, 2021; reviewed: September 21, 2023

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.  The same advice is true for other vaccines that are injected as well.

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

Last updated: April 28, 2023; reviewed September 21, 2023

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: September 21, 2023

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: September 21, 2023

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: September 21, 2023

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: September 21, 2023

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 mRNA from the vaccine causes production of COVID-19 spike protein. The protein (not the mRNA) is then processed by dendritic cells where pieces of the protein are put on the cell surface before the dendritic cell travels to the nearest lymph node and stimulates 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 (no longer used in the U.S.):

• 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: September 21, 2023

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. Some of them have subsequently been infected with SARS-CoV-2, and none of them have shown evidence of ADE.

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

Last updated: September 21, 2023

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 continues to circulate. These concerns take a few forms:

1. Compromised fertility in the vaccine recipient – Some concerns related to a placental protein, called syncytin-1. This protein is associated with the placenta during pregnancy. Online claims early during the pandemic 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: September 21, 2023

Clinical studies are underway to determine whether antiviral medications might help ease the lingering effects of COVID-19 infection, and studies of vaccination have suggested that by decreasing the severity of infection, fewer people experience long-term symptoms. However, vaccination does not appear to help ease symptoms in people who are already suffering the lingering effects of an infection.

Last updated: September 21, 2023

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: March 1, 2021; reviewed: September 21, 2023

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: September 21, 2023

A baby’s other vaccines will not protect them from 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: September 21, 2023

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.

Last updated: April 28, 2023; reviewed September 21, 2023