Archived COVID-19 Questions

Note to readers: the content on this page is not kept up to date and was accurate as of the date at the bottom of each question.

On this page, you will find archived answers to some of the previously common questions people asked about COVID-19 disease and vaccines. Just click on the question of interest and the answer will appear below it.

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You can also find information related to COVID-19 on our printable Q&A, "COVID-19 mRNA vaccines: What you should know" (also available in Spanish and Japanese) and our “Look at Each Vaccine: COVID-19 Vaccine” webpage.

I heard that steps were skipped to make a vaccine more quickly. Is that true?

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

Was the genome of the virus subject to peer review or FDA/CDC oversight?

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

Are young children susceptible to COVID-19, especially if a parent tests positive?

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:

Last updated: December 30, 2022

What is the difference between the first and second dose of the COVID-19 mRNA vaccine?

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

Can I get the second dose of COVID-19 mRNA vaccine in my other arm?

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

Why do kids need the COVID-19 vaccine since they don’t get that sick if they are infected?

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

Do I need another dose of the COVID-19 vaccine?

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:

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

If my child previously received two doses of COVID-19 vaccine, should they get a booster dose?

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.

Recent data suggest that longest-lasting, broadest protection against COVID-19 is afforded by either three doses of an mRNA vaccine or two doses of an mRNA vaccine plus a natural infection.

Last updated: April 25, 2023

Are some people at higher risk of having the clotting after the J&J vaccine?

In the U.S. women between 30 and 49 years of age have most often been affected by this condition compared with other groups; however, both men and women between 18 to 64 years of age have been affected.

About 1 of 500,000 vaccine recipients (with the adenovirus-based COVID-19 vaccine) will experience TTS.

Check out this infographic to see who is most often affected and how frequently this severe side effect occurs compared with others.

Last updated: Jan. 24, 2022; reviewed: December 23, 2022

What is the difference between TTS and CVST?

Thrombosis with thrombocytopenia syndrome, or TTS, is the name that has been given to the condition identified in a small number of individuals after receipt of the COVID-19 J&J/Janssen or AstraZeneca vaccines. TTS is distinct from cerebral venous sinus thrombosis, or CVST, because in TTS not all clots are associated with the large vessels of the brain. Some individuals had clots in their lungs, heart, intestines, spleen, or large blood vessels in their legs. Likewise, people with TTS have a low platelet count, called thrombocytopenia, which is not typical with CVST. However, CVST was originally used to refer to these cases because they closely resembled this previously defined condition.

Watch this short video in which Dr. Offit discusses the differences between typical blood clots and those occasionally reported following receipt of the COVID-19 adenovirus-based vaccine.

For more information about TTS, watch this short animation from the Melbourne Vaccine Education Centre.

Last updated: Jan. 24, 2022; reviewed: December 23, 2022


What are CVST and thrombocytopenia?

Cerebral venous sinus thrombosis (CVST) is a condition that causes blood clots in large vessels that drain blood from the brain. Although it is uncommon, the condition more often affects women between 20 and 50 years of age.

Thrombocytopenia is low numbers of cells called platelets. Platelets are cells that help our blood clot. When a person has this condition, they are at risk for bleeding since their body lacks the ability to efficiently stop the bleeding.

It is very uncommon for CVST and thrombocytopenia to occur at the same time, which is what makes this diagnosis following receipt of the J&J vaccine so unusual. Likewise, the clots have not just occurred in the large vessels near the brain in some of the affected individuals. This condition was subsequently named “thrombosis with thrombocytopenia syndrome,” or TTS.

Watch this short video in which Dr. Offit discusses the differences between typical blood clots and those occasionally reported following receipt of the COVID-19 adenovirus-based vaccine.

Last updated: Jan. 24, 2022; reviewed: December 23, 2022

What types of COVID-19 vaccines are being tested?

Several approaches to COVID-19 vaccines are currently being tested. They include both tried-and-true as well as novel approaches.

Here is a brief summary of these different strategies:

  • Inactivated vaccine — The whole virus is killed with a chemical and used to make the vaccine. This is the same approach that is used to make the inactivated polio (shot), hepatitis A and rabies vaccines.
  • Subunit vaccine — A piece of the virus that is important for immunity, like the spike protein of COVID-19, is used to make the vaccine. This is the same approach that is used to make the hepatitis and human papillomavirus vaccines.
  • Weakened, live viral vaccine — The virus is grown in the lab in cells different from those it infects in people. As the virus gets better at growing in the lab, it becomes less capable of reproducing in people. The weakened virus is then used to make the vaccine. When the weakened virus is given to people, it can reproduce enough to generate an immune response, but not enough to make the person sick. This is the same approach that is used to make the measles, mumps, rubella, chickenpox and one of the rotavirus vaccines.
  • Replicating viral vector vaccine — In this case, scientists take a virus that doesn’t cause disease in people (called a vector virus) and add a gene that codes for, in this case, the coronavirus spike protein. Genes are blueprints that tell cells how to make proteins. The spike protein of COVID-19 is important because it attaches the virus to cells. When the vaccine is given, the vector virus reproduces in cells and the immune system makes antibodies against its proteins, which now includes the COVID-19 spike protein. As a result, the antibodies directed against the spike protein will prevent COVID-19 from binding to cells, and, therefore, prevent infection. This is the same approach that was used to make the Ebola virus vaccine. (To see how viruses reproduce in cells, watch this short animation.)
  • Non-replicating viral vector vaccine — Similar to replicating viral vector vaccines, a gene is inserted into a vector virus, but the vector virus does not reproduce in the vaccine recipient. Although the virus can’t make all of the proteins it needs to reproduce itself, it can make some proteins, including the COVID-19 spike protein. No currently licensed vaccines use this approach.
  • DNA vaccine — The gene that codes for the COVID-19 spike protein is inserted into a small, circular piece of DNA, called a plasmid. The plasmids are then injected as the vaccine. No currently licensed vaccines use this approach.
  • mRNA vaccine — In this approach, the vaccine contains messenger RNA, called mRNA. mRNA is processed in cells to make proteins. Once the proteins are produced, the immune system will make a response against them to create immunity. In this case, the protein produced is the COVID-19 spike protein. The Pfizer and Moderna COVID-19 vaccines use this approach. Watch this short video in which Dr. Offit discusses how mRNA vaccines work.

Hear more about the types of vaccines being tested in this recorded event presented by Dr. Offit, Director of the Vaccine Education Center – Current Issues in Vaccines, December 9, 2020. This webinar series is supported by the Thomas F. McNair Scott Endowed Research and Lectureship Fund and co-sponsored by the PA Chapter, American Academy of Pediatrics and Wilkes University. (For healthcare providers wishing to obtain continuing education credits for viewing this recorded event, please review the continuing education information on this page.View answers to a series of questions asked during the event.

You can also visit the Vaccine Makers Project page, “The Coronavirus Pandemic – Answering Your Questions” for more details about the types of vaccines being studied. (See the “April 6” entry), or watch this CNBC interview with Dr. Offit for animations showing how some of these approaches work.

Last updated: Dec. 15, 2020

Which type of COVID-19 vaccine is most likely to work?

It is likely that more than one of these approaches will work, but until large clinical trials are completed, we won’t know for sure. Likewise, the different approaches may have different strengths and weaknesses. For example, mRNA or DNA vaccines are much faster to produce, but neither has previously been used to successfully make a vaccine that has been used in people. On the other hand, killed viral vaccines and live, weakened viral vaccines have been used in people safely and effectively for many years, but they take longer to produce.

In addition to differences in how long it takes to make different types of vaccines, each type may also cause the immune system to respond differently. Understanding the immune responses that are generated will be important for determining whether additional (booster) doses will be needed, how long vaccine recipients will be protected, and if one type offers benefits over another.

Last updated: Dec. 15, 2020

Is one of the COVID-19 vaccines expected be more effective for the elderly population? 

Each vaccine will need to be tested in various age groups; however, both the Pfizer and Moderna mRNA vaccines worked well in older adults.

Some of the traditional approach vaccines may not be effective in elderly populations. But if that is found to be the case, alternative versions may be explored, such as higher dose versions, as was done for influenza vaccines. Another possibility would be to deliver the SARS-CoV-2 spike protein with one or two powerful adjuvants, similar to what was done with the shingles vaccine, known as Shingrix®, which works well in the elderly.

Last updated: Dec. 15, 2020

What will be needed to obtain permission from the FDA for use of a coronavirus vaccine in the U.S.?

Vaccine manufacturers have to follow guidance provided by the Food and Drug Administration (FDA) while developing any coronavirus vaccine. This includes requirements to share information about how they determined that a vaccine is safe and that it works. Each company with a potential vaccine will need to provide data for review and information, so the FDA and other scientists can understand things like:

  • How the studies were designed
  • How many people were evaluated
  • How the testing to obtain the data was done

The FDA has also encouraged manufacturers to include people who represent the populations most affected by coronavirus in their studies, such as racial and ethnic minorities as well as older people and those with underlying illnesses.

Despite the shortened vaccine development timeline, the FDA has issued assurances that they will not approve a vaccine that was developed by sacrificing the standards for quality, safety, and efficacy that any other vaccine would need to meet. A group of vaccine manufacturers have also signed a pledge not to submit a COVID-19 vaccine before phase III studies have demonstrated that their candidate vaccine is safe and effective. The Biotechnology Innovation Organization (BIO) has also released an open letter to companies making COVID-19 vaccines regarding the standards that should be followed.

Last updated: Dec. 15, 2020

What are the phase III COVID-19 studies measuring to tell if a vaccine works?

The studies may be designed to measure slightly different things depending on the different companies:

  • Viral shedding – When people are infected with COVID-19, virus particles can be found in the secretions from their nose and mouth. Some studies are measuring whether vaccinated people have virus in these secretions, called viral shedding. With this approach, even if a person does not have symptoms, scientists can tell if the person was infected.
  • Protection against moderate or severe disease – In these studies, scientists evaluate people for specific symptoms of infection that are considered to represent more severe disease. By comparing the rates of these symptoms in people who were or were not vaccinated, they can tell if the vaccine protected more people from getting more severely ill. This is how the Pfizer and Moderna studies were done.
  • Some studies are evaluating both viral shedding and protection against moderate or severe disease.

Last updated: Dec. 15, 2020

What companies are working on a coronavirus vaccine?

More than 150 groups around the world are working on coronavirus vaccines, including in the United States, United Kingdom, Germany, India, China, Russia, and South Korea. To find out more about the vaccines being tested, download the file found on this page of the World Health Organization’s (WHO) website.

Last updated: Dec. 15, 2020

Who will get the vaccine first?

In most states and areas throughout the U.S., it is expected that frontline healthcare providers and residents of long-term care facilities will be prioritized, followed by sectors of the population at higher risk of suffering severe disease and death, such as essential workers and those with health conditions that put them at higher risk.

State and local health departments are basing their plans on recommendations made by the Centers for Disease Control and Prevention (CDC):

Last updated: Dec. 15, 2020

How long after testing positive for COVID-19, can I get my flu shot?

Talk with your healthcare provider about when you personally can get your influenza vaccine, but generally speaking, people can get a flu vaccine once they no longer have symptoms of COVID-19. 

Last updated: Dec. 15, 2020

Will getting the flu vaccine protect me against coronavirus?

No. Influenza viruses and coronaviruses are different, so the flu vaccine does not protect against coronavirus. You can learn more about the differences between these two viruses:

Even though flu vaccine does not protect against COVID-19, it is more important than ever to get an influenza vaccine this year:

  • For individuals — Preventing influenza infection will help people in two ways. First, since several of the symptoms of influenza and COVID-19 are similar, it will help with determining the cause of an infection this winter. Second, many people who die after having influenza actually die from a second infection. These second infections are called “opportunistic infections.” They are often the result of bacterial infections that capitalize on lung damage caused by influenza, leading to pneumonia. Given that COVID-19 also affects the lungs, it is possible that influenza infection could be complicated by COVID-19. It will take time to figure out the effects of these two infections on the lungs if they occur at or near the same time, so preventing or lowering the chance of influenza infection is important.
  • For healthcare systems — Every year, hundreds of thousands of people are hospitalized with influenza. Given that COVID-19 is already straining medical resources, it is possible that the impact of influenza and COVID-19 occurring at the same time will overwhelm healthcare systems, leading to both difficult decisions about how to best provide care and unnecessary deaths.
  • For communities — If both COVID-19 and influenza are spreading in communities and people are uncertain which they have, it could lead to excessive quarantines and shutdowns that contribute to an already fragile economic recovery. Likewise, from a public health standpoint, more people immunized against influenza will decrease its spread. So, overall, communities will be healthier, and community resources, such as EMTs, ambulances, public health, and related services will not be further overwhelmed by influenza.

Find out more and see tips for keeping your family healthy in the September 2020 Parents PACK article, “Coronavirus, influenza … Feel more in control this fall.”

Watch this short video in which Dr. Offit describes the importance of getting flu vaccine during the COVID-19 pandemic.

Last updated: Dec. 15, 2020

Does the pneumonia vaccine work against coronavirus?

Vaccines against pneumonia, such as the pneumococcal, Haemophilus influenzae type b (Hib), and meningococcal vaccines, are not likely to protect against COVID-19 disease.

However, the pneumococcal, Hib, and meningococcal vaccines prevent other serious infections. For more information about these vaccines and the diseases they prevent, go to:

Last updated: Dec. 15, 2020

Does the anthrax vaccine protect against coronavirus?

The anthrax vaccine has not been tested for any ability to provide protection against COVID-19 disease, nor would it be expected to do so.

The anthrax vaccine is not routinely recommended in the U.S., but some high risk groups of people are recommended to get it.  Find out more on the “A Look at Each Vaccine: Anthrax Vaccine” webpage.

Last updated: Dec. 15, 2020

Can DNA vaccines change the DNA of a person?

DNA vaccines are among the types being tested to prevent COVID-19. DNA vaccines may offer an advantage to other types of vaccines because they do not need to use the virus to produce an immune response. Other DNA vaccines in development (including for influenza, HPV, and HIV) have been extensively tested in animals and people and shown to be safe.

Some people are concerned that DNA vaccines could change people’s existing DNA. While it is the case that the DNA from the vaccine is treated the same as human DNA in the cell, two points are worth considering. First, DNA vaccines have not historically worked as well as other types because it is difficult to get enough DNA introduced to the vaccine recipient to make a strong immune response. Second, even when it is introduced successfully, vaccine DNA only enters a few cells, which then produce viral proteins that, once released from the cell, activate the immune system to generate immunity.

As such, concerns related to a person’s DNA being altered by DNA vaccines are theoretical concerns without evidence. Since all vaccines are required to go through extensive testing before approval, the same standard will be applied to DNA vaccines as will be applied to any other COVID-19 vaccine.  Only then will we know whether DNA vaccines will be useful in protecting against COVID-19.

Last updated: Dec. 15, 2020

How can our family safely celebrate the holidays?

The November 2020 issue of Parents PACK offered tips for families as they plan for the holidays. Topics addressed include advance planning, travel and lodging, and during- and after-event precautions.

Check out the article on this page or download a PDF for sharing.

Last updated: Dec. 15, 2020

Will there be more shutdowns?

Stay-at-home orders have been used as a way to decrease the spread of COVID-19; however, it is unlikely that the country will experience widespread shutdowns like those implemented in the spring of 2020. Rather, it is expected that locally targeted public health measures will be “turned up” and “turned down,” depending on situations in each community.

With this said, the virus is currently spreading so rapidly that we may see many communities or larger geographic areas concurrently implementing measures to slow the spread, particularly as the holidays may increase an already “out-of-control” spread. Indeed, the CDC recently asked Americans not to travel during the Thanksgiving and Christmas holidays. The best way to decrease the need for restrictions is for communities to ban together when it comes to wearing masks and social distancing, particularly until enough people have been immunized to make it difficult for the virus to find susceptible people in the community.

Last updated: Dec. 15, 2020

I heard something about MMR vaccine helping against COVID-19. Can you tell me what that is about?

Some have proposed that giving people a live weakened vaccine, such as MMR (measles, mumps, and rubella) vaccine, might lessen the severity of coronavirus disease, including swelling in the lungs and sepsis, an infection in the bloodstream. While there is some evidence that live weakened vaccines can induce immunity that can protect against other infections, any protection would not be specific. At this time, use of MMR, or any other live weakened vaccine, has not been studied as a way to prevent the complications of coronavirus in either animals or people, so it is not recommended. Further, because the protection would not be specific or long-lived, it would not be as useful as COVID-19 vaccines.

For more information about MMR vaccine, go to "A Look at Each Vaccine: Measles, Mumps and Rubella (MMR) Vaccine." 

Last updated: Dec. 15, 2020

Is there a cure for coronavirus?

No cure for coronavirus is available.

People who are mildly ill and recovering at home
Rest and drinking plenty of fluids are most important for people with mild cases of COVID-19. While over-the-counter medications can be used to make a person more comfortable, it is important to realize that often symptoms are the result of the immune response to infection. For example, fevers make the body a less “comfortable” environment for the virus to reproduce and allow the immune system to work better. So, treating fever can prolong illness. To learn more about fever, check out this downloadable question-and-answer sheet, or to find out more about fever and vaccines, check out this webpage.

Those recovering at home should be in touch with their healthcare provider for specific medical recommendations, and anyone having trouble breathing, or who quickly takes a turn for the worse, should seek immediate medical care. A monoclonal antibody treatment, called bamlanivimab, was recently approved for treatment of mild-to-moderate disease in those with high risk conditions.

It is also important for those with mild illness to still isolate from others in the home as they can still spread the virus.

People who are severely ill and require hospitalization
Healthcare providers have been learning more about ways to treat people who become severely ill. One medication, Remdesivir, has been shown to shorten the length of infection. Similarly, a drug called dexamethasone (a steroid) has been shown to be of benefit for people with pneumonia caused by SARS-CoV-2.

Treatment of hospitalized patients varies based on the symptoms and complications each patient experiences.

Last updated: Dec. 15, 2020

How did the new coronavirus form?

In late 2019, a novel coronavirus, SARS-CoV-2, formed. This new virus had two important characteristics. First, it could cause severe disease and death in people. Second, it could easily spread from one person to another. Together with the fact that no one in the world was immune, these characteristics set the stage for the COVID-19 pandemic that quickly changed current life as we knew it.

As scientists around the world have diligently worked to help us understand this virus and the disease it causes, it is important to realize that we will continue to learn more about this virus for many years to come. But, we have learned some important things already:

  • For many people, disease will be mild. Some people will not experience any detectable symptoms. But, this is not true for everyone, and, as with other infections, science has not evolved to a point that we can predict who will become severely ill, experience complications, or die. This means, everyone should take the virus seriously — if not for themselves, then for those around them.
  • In some cases, groups of people are known to be at greater risk of suffering severe COVID-19. These include older adults and people of color as well as sub-groups of the population, like those with heart and lung disease, obesity, type-2 diabetes, and pregnant women.
  • And, for some people symptoms last extremely long, and lingering, long-term effects may also result, although this is another area that healthcare providers and scientists are still working to understand.
  • Finally, while healthcare providers are learning more every day about how to treat COVID-19, many medications and treatments are still being studied.

For these reasons, and because prevention is better than treatment, COVID-19 vaccines are essential to returning to some sense of normalcy. But, we can also work together using important public health practices to limit the illness, deaths, and societal damage SARS-CoV-2 seeks to sow.

Last updated: Dec. 15, 2020

What are the ways that we can prevent spread of COVID-19?

Coronaviruses spread through respiratory secretions, like saliva produced during coughing, speaking, and singing, and nasal secretions producing during sneezing or from a runny nose. Droplets from coughs or sneezes are often considered “large droplets,” meaning they don’t hang in the air for long. But, in some cases, small virus-containing droplets can remain in the air for longer periods of time. These are sometimes referred to as “aerosolized droplets,” and they are more contagious. Viruses present in either of these sized droplets can enter another person’s eyes, nose, or mouth and infect cells that line that person’s nose, throat, lungs, blood vessels, and intestines. As such, we can protect ourselves and decrease spread in these ways:

  • Handwashing – Because we touch so many things, we almost always have germs on our hands. If these viruses or bacteria are still infectious, we can easily inoculate ourselves, particularly if we touch our eyes, noses, or mouths with unclean hands.
  • Social distancing – Because respiratory droplets travel some distance after leaving our mouths or noses, staying further apart decreases the chance for particles containing viruses to then be breathed in or land on us. The speed and force with which a particle leaves the body determines how far it can travel before landing somewhere. So, droplets from a sneeze will travel further than those spread during speaking. While a particular distance is not guaranteed to protect someone, the Centers for Disease Control and Prevention (CDC) has recommended 6 feet. In enclosed spaces, like small rooms or cars, the chance for exposure increases; therefore, it is important to increase ventilation, by opening windows and not using recirculated air when possible.
  • Masks – While masks have been somewhat controversial, they do help to decrease the spread of this virus, particularly because it can spread through “small droplets,” which means you do not need to see the droplets for them to spread the virus. Masks primarily decrease the spread of virus by the person wearing them, but they may also somewhat reduce the chance of being infected.

Find out more about masks:
How to properly put on, take off, and clean masks, October 2020 Parents PACK
Images of correct and incorrect ways to wear masks as well as information about studies of different mask materials, Vaccine Makers Project News & Events, September 30, 2020

  • Disinfecting surfaces – When respiratory droplets land on surfaces, the viruses they carry remain infectious for hours to days, depending on the characteristics of the virus, the type of surface on which they land, and the surrounding environment. Because the virus can be spread in droplets not visible to the eye, regularly cleaning surfaces that are touched often or around which people often pass is important to decreasing viral spread.
  • Staying home when sick – As with other respiratory viruses, anyone with symptoms should stay away from others to decrease spread. If someone with symptoms coughs or sneezes, their respiratory secretions are likely to have even greater amounts of virus than someone without symptoms. Therefore, they may infect more people or the small number of people they infect may get a bigger inoculation, which could lead to more severe disease. In the case of COVID-19, people who know (or think) they have been exposed are also being asked to isolate because it is estimated that about 4 of 10 people who are infected do not develop symptoms.

Last updated: Dec. 15, 2020

What is a challenge study?

To see whether a vaccine works, scientists have to determine whether people who got the vaccine are less likely to get sick with the disease than those who were not vaccinated. They can do this in one of two ways:

  • If the pathogen is spreading in the community, they can see if fewer vaccinated people get sick.
  • But, if the pathogen is only spreading at low levels, they may not be able to tell because differences between vaccinated and unvaccinated people could simply be due to differences in exposure to the virus. In this situation, challenge studies can be used.

A “challenge study” is one that includes intentional exposure to the pathogen as part of the research plan. For example, a study volunteer might get vaccinated and a month after receiving the last dose be intentionally exposed to the pathogen. After exposure, the volunteer would be monitored for both symptoms of illness and the presence of a memory immune response, such as through the measurement and typing of antibodies in a blood sample. Challenge studies have been used in the past to determine the effectiveness of various influenza vaccines.

Challenge studies offers a few benefits:

  • As mentioned, they do not need to rely on disease spreading in the community to see whether the vaccine works.
  • Scientists know when the person was exposed to the pathogen.
  • They can control the exposure, so that the person is not likely to get severely ill if the vaccine did not work as expected.

However, challenge studies can have ethical and technological drawbacks that must be considered before being implemented:

  • By definition, this kind of study means volunteers are being intentionally exposed to a pathogen. If the vaccine does not work, some people could become severely ill, or even die.
  • Currently, for COVID-19, we have limited treatment options, so treating anyone from a study who becomes ill would be further hindered by these limits.
  • Scientists have an important responsibility when they choose the challenge strain of the pathogen. They need to choose a strain that will cause the immune system to respond, but not one that will cause severe illness. In the current COVID-19 situation, much remains to be learned about this pathogen, which complicates the choice of a challenge strain.
  • Also, under natural conditions, people will be exposed to different burdens of virus. As a general rule, the greater the amount of virus the worse the symptoms. Challenge studies, however, are likely to use only one dose of virus.

Watch this short video in which Dr. Offit discusses challenge studies.

Last updated: Dec. 15, 2020

What are the different phases of clinical trials (phase I, II, and III studies)?

Vaccine development typically follows a progression of increasingly larger studies to limit risk while learning about the potential vaccine. Every phase monitors safety of the potential vaccine, but each phase also has additional specific goals:

  • Phase I studies typically include fewer than 100 healthy adults and are designed to figure out if the potential vaccine generates an immune response. Watch a short CNN interview in which Dr. Offit discussed how to think about Phase I results (July 21, 2020).
  • Phase II studies typically include a few hundred healthy adults and are designed to figure out the optimal vaccine dose and vaccine production specifications and tests.
  • Phase III studies typically include tens of thousands of participants. Ideally, these participants represent the population of people who will be recommended to get the vaccine. These studies evaluate whether the vaccine works in the intended population, and because of the number of people who receive the potential vaccine, they are important for detecting side effects that may occur infrequently, and, therefore, might not have been found in the earlier phases. These are the last studies completed before a potential vaccine can be licensed. Early phase III trials of the COVID-19 vaccine were designed to include about 20,000 people who got the vaccine and 10,000 people who get a shot that doesn’t contain the vaccine (i.e., a placebo). Some companies, however, might do smaller phase III trials.
  • After approval, scientists continue to monitor vaccines in “post-licensure studies.” In this way, they can quickly become aware of any previously undetected issues of concern, so that vaccinations can be halted if necessary. The Vaccine Safety Datalink in the United States is designed to quickly pick up a safety problem once the vaccine has been administered to hundreds of thousands or millions of people.

For more details about each of these phases, visit the VEC’s webpage, “Making Vaccines: Process of Vaccine Development.”

For more details about the Vaccine Safety Datalink, visit the CDC’s webpage, “Vaccine Safety Datalink (VSD).”

Last updated: Dec. 15, 2020

How long does each phase of vaccine testing last?

The amount of time for each phase of vaccine development varies based on a variety of factors related to the vaccine being tested, the disease, and the way the studies are designed. But, generally speaking, phase I trials take about 1 to 2 years, phase II trials take 2 or more years, and phase III trials take 3 to 4 years to complete.

In the case of COVID-19 vaccines, this timeline was shortened by combining the phases of trials, decreasing the number of participants in the early phases of the trials, and adding tremendous resources to allow for faster completion. For example, the government invested in some of the potential vaccine candidates to allow for building the manufacturing facilities and making vaccine doses before it is was known whether the vaccine worked. For example, because the Pfizer mRNA vaccine worked and was safe, doses could be distributed shortly after the vaccine was approved for use, but if the vaccine did not work, the doses would have been discarded.

You can find out more here:

Last updated: Dec. 15, 2020

When will I be able to get a vaccine against COVID-19?

The U.S. Centers for Disease Control and Prevention (CDC) created a framework, informed by the efforts of other groups, to advise states regarding the equitable distribution of limited supplies of COVID-19 vaccines:

With this information states, territories, and local health departments each created their own prioritization plans and submitted them to the CDC. You can see the executive summaries of each plan on this page of the CDC’s website. However, these plans are continually being updated and adapted as more information becomes available. Therefore, you may wish to visit the website of your state, territory, or local department of health to find out more about what is happening in your area.

Last updated: Dec. 15, 2020

When will there be enough vaccine for everyone who wants to be vaccinated to get a COVID-19 vaccine?

While we cannot know for sure, if both the Pfizer and Moderna vaccines are provided in the quantities they estimate producing, it is expected that by summer or fall of 2021 most people who want a COVID-19 vaccine should be able to receive it, as long as they do not have a contraindication that prevents them from receiving these versions.

Likewise, it is possible that vaccines produced by Johnson & Johnson and AstraZeneca will become available during 2021 that could change these projections.

Last updated: Dec. 15, 2020

Since the vaccine has to be stored at such a cold temperature, what temperature will it be when it is given?

The vaccine will be thawed, allowed to come to room temperature, and diluted with a salt solution before being administered. Providers will be given specific instructions to ensure that how they handle the vaccine will not affect its effectiveness.

Last updated: Dec. 15, 2020

How do we know if COVID-19 vaccines are safe?

Given that COVID-19 vaccines were made more quickly than other vaccines, it is understandable to be concerned about their safety, but the following can provide reassurances:

  • Phase III trials for COVID-19 vaccines have been as large as those for other vaccines, including tens of thousands of participants. While these trials may not uncover rare adverse events (that occur in the millions), we can be comfortable that these trials were large enough to detect any major safety concerns.
  • For each new vaccine, the data from these large phase III vaccine trials will undergo several rounds of review by different, independent groups of experts in immunology, statistics, infectious diseases, virology, and vaccinology:
    • The trials are coded, so that the manufacturers do not know who got vaccine and who got placebo. During this time, an independent group of experts monitor the data to make sure that no concerning developments occur with trial participants.
    • Once the company submits the data to the Food and Drug Administration (FDA), an advisory committee of independent experts, called the Vaccine and Related Biologics Product Approval Committee (VRBPAC) reviews the data to evaluate vaccine safety and effectiveness. This committee provides advice to the FDA before a vaccine can be accepted.
    • Once a vaccine is approved by the FDA, the data undergo a third round of review, by yet another committee of experts. This group, called the Advisory Committee on Immunization Practices (ACIP), reviews the data and make recommendations to the Centers for Disease Control and Prevention (CDC) regarding who should or should not get the vaccine and when, based on the data.

As a result, by the time a vaccine can be given to any individual, the results of the phase III trials have been reviewed and discussed regarding their scientific merit by more than 50 independent experts, in addition to the scientists at the companies and their own trial advisory groups.

Watch this short video to hear Dr. Offit discuss how we will know a COVID-19 vaccine is safe.

COVID-19 vaccines will be approved using the FDA’s “Emergency Use Authorization (EUA)” process, rather than the typical “Biologics Licensing Application (BLA)” process, due to the emergency we face resulting from the pandemic. However, the main difference between these processes from the point of view of vaccines is that under the EUA process, the vaccines will be approved more quickly, so they can get to people faster. As such, companies will need to continue monitoring the trial participants and submit subsequent findings to the FDA. In reality, companies usually continue following participants, but the time difference here is that rather than having studied the vaccine for several years, it has only been studied for several months; therefore, the additional monitoring will be critical to continuing to understand the durability of immune responses to the vaccine.

To find out more about the emergency use authorization compared to the typical vaccine approval process, watch this interview with Dr. Offit posted by Medscape (October 27, 2020).

Finally, once a vaccine is approved and individuals are deciding whether to get it, they will also have access to information that summarizes the clinical trial findings, particularly related to any side effects found during the trials and who should or should not get the vaccine. In the U.S., a document called a Vaccine Information Statement, or VIS, is legally required to be presented before every dose of vaccine is given. While a COVID-19 vaccine will be approved outside of the normal process, because of the EUA protocol, a VIS, or similar document, should still be made available to provide the information that individuals will need to make informed decisions based on the known risks and benefits.

To read more about the history of the VIS, see “History of Vaccine Information Statements,” produced by the Centers for Disease Control and Prevention.

Last updated: Dec. 15, 2020

If I am currently staying in a state that is not my permanent residence, can I get the vaccine?

We recommend that you inquire with the Department of Health in the state that you are in to find out more specific information. While most states are not checking residency, a few are. You can find each state’s COVID-19 information on this page of the Vaccinate Your Family website.

Last updated: Mar. 3, 2021

Once I have been vaccinated against coronavirus, am I exempt from restrictions?

Fully vaccinated individuals (at least two weeks after the last dose in the series) should still follow public health measures in public, in groups that may have vaccinated and unvaccinated individuals, or in situations with individuals from multiple households. However, vaccinated individuals can gather with those from another home who are also vaccinated can gather without masks in their homes for meals.

Some have wondered why we still need to practice recommended public health measures as more individuals are vaccinated. Unfortunately, a large percentage of the population will need to be immune to slow or stop the spread of the virus. Two factors are important for understanding why:

  • While the vaccines appear to be highly effective at preventing disease, they might not prevent asymptomatic infection, meaning vaccine recipients might still be able to get infected, but not have symptoms and, therefore, unwittingly spread the virus. The companies are doing additional studies to better understand whether this is the case.
  • Scientists estimate that to control COVID-19, about 8 of every 10 people will need to be immune. Given that the U.S. population is more than 330 million people, this means that almost 200 million of them will need to be immune to reach this goal either from vaccination or natural infection.

For these reasons, there will still be some period of time during which other measures, such as masks, social distancing, and other public health measures, will be required to slow or stop the spread of the virus. And, because we won’t know who might still be able to be infected after vaccination or previous illness, everyone will be asked to comply. Watch Dr. Offit discuss the continued need for masks and social distancing in this short video.

Last updated: Apr. 23, 2021

Will annual or booster doses of COVID-19 vaccines be needed?

While we are still learning how long immunity lasts after illness or vaccination, data have started to emerge suggesting that some individuals may require a third dose (See “Do I need a third dose of the COVID-19 vaccine?”). Compromised or waning immunity may cause the need for additional doses.

Further, the increased presence of variants might also affect whether additional doses will be needed. This is one reason scientists are anxious to get as many people vaccinated as possible — the more the virus can replicate in people, the greater the chance that it will change in a way that allows it to spread widely — even among people who have been vaccinated. With this in mind, scientists are continuing to monitor variants and the ability of currently available vaccines to protect against them.

Last updated: Nov. 10, 2021

What do you know about the halt on the J&J vaccine?

Federal health officials recommended a pause on the Johnson & Johnson (J&J) COVID-19 vaccine after six women developed blood clots in their brain up to 3 weeks after their vaccine. The blood clots in the brain are called cerebral venous sinus thrombosis (CVST). While a few of these women had an underlying health condition, no pattern of pre-existing conditions emerged. These cases are similar to those caused in European countries by the AstraZeneca (AZ) vaccine, which is not currently used in the United States. The J&J and AZ vaccines are similar in that both use an adenovirus vector to induce immunity to the SARS-C0V-2 surface protein, but they use different adenovirus vectors. The J&J vaccine uses a human adenovirus, and the AZ version uses a chimp adenovirus.

The pause was recommended for three reasons:

  • First, the FDA and CDC needed time to review these cases and collect as much information as they could to figure out what was happening.
  • Second, they needed to update healthcare providers across the country on what to watch for in their patients. In this way, physicians can properly diagnose and treat the condition. This condition is very rare and treating it with blood thinners could make this situation worse.
  • Third, the pause allowed for the public to quickly become aware of the potential side effect, so that people who got the vaccine and those around them would be more likely to recognize the need to seek medical attention if they experience symptoms.

After evaluating all available sources of information, including the scientific literature, other types of clotting events in vaccine recipients, U.S.-based safety systems and European safety data, the CDC determined that the benefits of the J&J/Janssen vaccine clearly and definitively outweigh the risks.

As of early May 2021, individuals should be aware of the following:

  1. Twenty-eight cases occurred after nearly 9 million people received the J&J vaccine, so the risk is extremely low, about 3 in a million. Twenty-two cases have occurred in women and 6 cases have occurred in males.
  2. The cases occurred in individuals between 18 and 64 years of age; however, women between 30 and 49 years of age are at the highest risk of experiencing this severe side effect. To date, three people have died from this side effect.
  3. Most cases have occurred between 1 and 2 weeks after vaccination, but some have been identified as early as 3 days and as late as 15 days following vaccination. Therefore, anyone who gets the J&J vaccine should monitor for symptoms up to 3 weeks after getting vaccinated.
  4. This episode demonstrates the robustness of the vaccine safety monitoring systems in place in the U.S. as the systems detected something happening at a rate of about 1 in a million in less than 6 weeks.

No cases of this condition have been identified following receipt of either mRNA vaccine. At this time almost 250 million doses of the two mRNA vaccines have been administered in the U.S.

Watch this short video in which Dr. Offit discusses the differences between typical blood clots and those occasionally reported following receipt of the COVID-19 adenovirus-based vaccine.

Last updated: May 13, 2021

Is it okay for people in the same house to get different types of COVID-19 vaccine or should they get the same brand?

It is fine for people in the same home to get different brands or versions of COVID-19 vaccine.

Last updated: Mar. 18, 2021

How well do the COVID-19 vaccines work?

All three COVID-19 vaccines approved for use in the U.S. work well.

mRNA vaccines: More than 9 of every 10 people vaccinated during the clinical trials were protected from disease. Likewise, a study of vaccine performance in the community also found that 9 of 10 individuals are protected. Read a summary of the study.

Adenovirus-related vaccine: While only about 6 or 7 of every 10 people vaccinated during the clinical trials were protected from disease, the study found that 8 or 9 were protected from severe disease and all were protected against hospitalization by one month after vaccination. Also, studies of the mRNA and adenovirus vaccines were done on different populations, making it difficult to compare these vaccines. Whereas all of the mRNA studies were done in the United States, the adenovirus vector vaccine (Johnson & Johnson/Janssen) studies were performed in Latin America, South Africa and the United States. Because the viral strains circulating in South Africa and Latin America were different from those circulating in the United States, it’s difficult to directly compare the relative efficacies.

Last updated Apr. 23, 2021

How well does the COVID-19 vaccine work in adolescents?

The clinical trial measured two things to evaluate how the vaccine worked:

  1. Disease – While 18 participants (12-15 years of age) in the placebo group got COVID-19 at least seven days after having the second dose of the vaccine, none in the vaccinated group were infected. This represents 100% efficacy. In the 5- to 11-year-old group, three vaccinated children got COVID-19 compared with 16 children in the placebo group; this represents a 90.7% efficacy.
  2. Immune response – Now that we know what antibody levels adults experience following vaccination, studies can compare the levels in other groups to see if the vaccine works as well. These are often referred to as “non-inferiority studies,” meaning the vaccine is being tested in the study group to make sure it works at least as it does in another group previously studied. In the Pfizer study, average neutralizing antibody responses were similar in 5- to 11-year-olds compared with older children and young adults even though the dose was lower. While we still do not know if a certain level of virus-neutralizing antibodies indicates that an individual is protected against infection, we can be confident that vaccinated children respond equally well compared with older children and young adults.

Last updated: Nov. 10, 2021

Could the mRNA vaccines from Pfizer and Moderna cause the same clotting problem as the J&J vaccine did?

The Johnson & Johnson (J&J)/Janssen COVID-19 vaccine is an adenovirus vector vaccine, which is different from the Pfizer and Moderna mRNA vaccines. At the time of the J&J/Janssen pause, more than 182 million doses of the mRNA vaccines had been administered and no cases of thrombosis with thrombocytopenia syndrome, or TTS, had been reported. Three people out of about 85 million doses of Moderna had blood clots, but they did not have low platelets. The number of blood clots experienced by those who got the Moderna vaccine would be expected based on the background rate of clotting in the general population.

Last updated: May 13, 2021

How much does the coronavirus vaccine cost?

Coronavirus vaccines are free; however, the Federal Trade Commission (FTC) has warned of scams in which people are charging for vaccines. Read more here.

Also, of note, while the vaccines are free, insurance companies may have to cover the cost of administering the vaccine. You should not, however, be charged any out-of-pocket fees when you go for your vaccine.

Last updated: Apr. 23, 2021

We don't talk about "herd immunity" for protection against influenza or other common viral infections, so why is it discussed so much with COVID?

Herd immunity is a concept used in public health to describe a situation in which the more people in a community immune to a particular pathogen, the fewer people available for that pathogen to infect. As the infectious agent spreads through a community, it has more trouble finding susceptible people if most of those around them are immune. In this manner, we rely on herd immunity for viruses, such as measles, rubella, polio, and chickenpox, among others, even if we are not having conversations about it. Influenza is more difficult because the virus changes so much from one year to the next and as such, vaccination does not offer long-term protection.

Related to COVID-19, herd immunity has been discussed more frequently for a couple of reasons. First, because this is a completely new virus, no one had pre-existing immunity. People can become immune to SARS-Co-V2, the virus that causes COVID-19, in two ways — through disease or through vaccination. By monitoring how many people are immune relative to the entire population, public health officials can offer informed guidance related to easing restrictions meant to stem spread of the virus.

Herd immunity can only be induced by vaccination. Never in history has any virus infection been eliminated because of immunity induced by natural infection.

Last updated: Mar. 1, 2021

When will COVID-19 vaccines be available for children younger than 5 years of age?

COVID-19 vaccine trials are in progress for children younger than 5 years of age.

Find out more about COVID-19 clinical trials in children in this Parents PACK article, including how the trials are being done, what will be learned and more.

Last updated: Jan. 20, 2022

If a person has allergic reactions to a food or medication, can they get the vaccine?

People with severe allergies to a COVID-19 vaccine ingredient (see list here) or a previous dose of COVID-19 vaccine should not get that type of COVID-19 vaccine (mRNA or adenovirus-based). They may be able to get the alternative type after consultation with an allergist or immunologist. Individuals with a known allergy to polysorbate should not get the COVID-19 vaccine made by Johnson & Johnson/Janssen.

People with immediate allergic reactions to an injectable medication can most often get the COVID-19 vaccine; however, they should remain at the site where they were vaccinated for 30 minutes of observation, instead of the 15 minutes that the general public is recommended to wait. Anyone with this type of allergy who has questions or concerns should discuss the situation with their healthcare provider to assess the potential risks and benefits of receiving the COVID-19 vaccine. 

People who have had an anaphylactic reaction to anything else (medications, foods, bees, etc.) are allowed to get the COVID-19 vaccine, but should remain at the site where the injection was given for 30 minutes, instead of the 15 minutes that the general population are recommended to wait.

The CDC published information about allergic reactions that caused anaphylaxis after almost 2 million doses of the Pfizer vaccine were given. They estimate that about 30% of the population has allergies. However, only 21 anaphylactic allergic reactions occurred in those 2 million vaccine recipients. Of these 21 people, 17 of 21 had previously identified allergies, but 4 of 21 had no previously identified allergies at all. Of those who had allergies, no significant pattern emerged, suggesting that there is not a causal association between allergies (or specific allergies) and an anaphylactic reaction to the vaccine. Further, since millions more doses have been administered, this rate of allergic reaction has not continued, suggesting that the likelihood of having an allergic reaction following receipt of the COVID-19 vaccine is not likely to differ from background rates.

If a person with history of allergies continues to have concerns about whether or not it is safe to get the COVID-19 vaccine, they should contact their primary care provider or allergist, who has the benefit of their complete medical history and will, therefore, be in the best position to discuss any potential risks and benefits for that individual.

Last updated: May 27, 2021; reviewed: Jan. 20, 2022

What should I do if I had the J&J vaccine?

People who had a single dose of the J&J/Janssen vaccine are recommended to get a second dose at least 8 weeks after the first dose.

Related to side effects

If you had the J&J/Janssen vaccine within the last 3 weeks, although the risk is low, you should monitor yourself for unusual symptoms, including severe headache, severe abdominal pain, unexplained leg pain, or shortness of breath, which may result from TTS, or muscle weakness or paralysis, which may result from GBS. If you develop unusual symptoms, you should seek medical attention and be certain to tell the healthcare provider the date you received the J&J/Janssen vaccine. For TTS, the physician can very quickly determine whether your problem is related to the vaccine by performing a simple complete blood count. If the platelet count is extremely low, the symptoms might be related to the vaccine. We would also recommend registering for v-safe, the CDC’s vaccine monitoring system if you have not done so already.

If you had the J&J/Janssen vaccine more than 3 weeks ago, you are extremely unlikely to experience either thrombosis with thrombocytopenia syndrome (TTS) or Guillain-Barré syndrome (GBS).

Last updated: Nov. 10, 2021; reviewed: Jan. 10, 2022

Should I stop using my birth control if I got the J&J vaccine?

It is not necessary to stop taking birth control pills. Individuals affected by thrombotic thrombocytopenic syndrome (TTS), which is an unusual combination of low platelet count (thrombocytopenia) and clotting (thrombosis) did not share common medical histories, such as use of birth control pills; therefore, stopping usage would not change your risk for TTS.

Last updated: Apr. 23, 2021; reviewed: Jan. 20, 2022

What if I can’t get the second dose 21 (Pfizer) or 28 (Moderna) days after the first dose?

The Centers for Disease Control and Prevention (CDC) allows for a 4-day grace period when assessing on-time receipt. This means the following ranges of days are considered “on-time” for receipt of the second dose:

  • Pfizer vaccine: 17 to 25 days after the first dose
  • Moderna vaccine: 24 to 32 days after the first dose

People should try to get the second dose during this period or as soon after as possible. However, if your second dose is given later than this, you do not need to restart the vaccine. It is important to note that during the clinical trials the first dose did not protect as many people as were protected after the second dose, so if you are exposed to SARS-CoV-2 during the delay, you may or may not have enough immunity to prevent you from experiencing symptoms. The timing of your third dose would be based on when you got the second dose.

Last updated: Jan. 24, 2022

Can I drink alcohol after getting the COVID-19 vaccine?

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

Last updated: Dec. 31, 2020; reviewed: April 27, 2023

How many doses of a COVID-19 vaccine will be needed?

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 (spring 2023), here is a summary to help you sort through the recommendations:

General information

  • At this time, recommendations are based on age, COVID-19 vaccination history, and immune status. Because updated recommendations are currently being developed for those considered moderately or severely immune compromised, those individuals should discuss their situation with their healthcare provider.
  • The preferred vaccines are the bivalent mRNA vaccines produced by Pfizer and Moderna. As such, the below information focuses on the use of these vaccines. Those who require Novavax or J&J/Janssen should speak to their healthcare provider or the providers at the vaccination location about their dosing needs.

Previously unvaccinated

  • If patient is 6 months to 4 years of age, they should get two doses of Moderna (25 micrograms) or three doses of Pfizer (3 micrograms; maroon cap). Moderna doses should be separated by four to eight weeks, and Pfizer doses should be separated by three to eight weeks between the first two doses and at least eight weeks between the second and third doses.
  •  If patient is 5 years of age, they should get two doses of Moderna (25 micrograms) separated by four to eight weeks or one dose of Pfizer (10 micrograms; orange cap).
  • If patient is 6 to 11 years of age, they should get one dose of either Moderna (25 micrograms) or Pfizer (10 micrograms; orange cap).
  • If patient is 12 years or older, they should get one dose of either Moderna (50 micrograms) or Pfizer (30 micrograms; gray cap).
  • If patient is 65 years or older, they have the option to receive a second dose of the same brand received previously. At least four months should pass before the second dose is administered.

Previously received one or more doses of COVID-19 vaccine

Most who had a single dose of bivalent COVID-19 vaccine do not require additional doses at this time. The exceptions are:

  • Children between 6 months and 4 years of age who had one dose of Pfizer’s monovalent mRNA vaccine (3 micrograms) should get two additional doses of Pfizer’s bivalent version (3 micrograms). The first bivalent dose should be given three to eight weeks after the monovalent dose, and the second bivalent dose should be given at least eight weeks after the first bivalent dose.
  • Adults 65 years of age and older who had one dose of either Pfizer’s or Moderna’s bivalent mRNA vaccine can get a second dose of the same brand at least four months after the last dose.

Others who have not received any doses of a bivalent COVID-19 vaccine are recommended to get one age-appropriate dose of bivalent vaccine, regardless of whether they received an mRNA, adenovirus or protein-based vaccine. If they previously received an mRNA version, the same brand previously received should be used for the bivalent dose. The bivalent vaccine should be given at least eight weeks after the last monovalent dose with the exception of children 6 months to 5 years of age who only had a single dose of monovalent Moderna-brand vaccine (25 micrograms); these children should get the bivalent dose four to eight weeks after the last monovalent dose.

Last updated: April 27, 2023

Could taking two different vaccines boost the effectiveness?

Currently, the Centers for Disease Control and Prevention (CDC) recommends getting all primary doses of the same mRNA vaccine whenever possible.

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

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

Last updated: April 27, 2023

If I had dermal fillers, can I get the COVID-19 vaccine?

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 for Dermatologic Surgery.

Last updated: April 28, 2023

If I need a dental procedure, can I get the COVID-19 vaccine, or should I delay my procedure?

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: April 28, 2023

How can I know about COVID-19 disease in my community or where I am traveling?

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. Most of these tools are no longer in operation; however, they could be restarted if a need arises in the future. For now, you can monitor CDC’s “COVID Data Tracker,” where a section on the page allows individuals to enter a state or county to see COVID-19 infection and vaccination data.

Last updated: June 23, 2022; reviewed: April 28,2023


Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.