Frequently Asked Questions

Vaccine Booster FAQs

A: Booster doses of a vaccine are very common in the field of vaccines. Boosting means getting another dose of vaccine at a different time after the initial dose(s) of a vaccine, which scientists call the “priming” stage. It’s like wall paint: priming coat, maybe a second priming coat, followed by another boosting coat. For vaccines, the boost is weeks, months, or years later, rather than a day or two for wall paint. Like a second coat of paint, the boosting improves upon the priming. 

Most vaccines we get have boosting components. For hepatitis B, babies get the vaccine at 1, 2 and 6 months. The 6-month vaccine is the boost. For the measles vaccine, children get it at about 12 or 15 months and then the boost 5 years later when they enter elementary school. For tetanus, you get a booster every 10 or so years. 

The additional doses given to immunocompromised individuals are to improve their immune response to the initial vaccine series. They often do not build enough, or any, protection against the virus from the original vaccination.

A: For mRNA vaccines (Pfizer and Moderna), the FDA and ACIP both recommend boosting for all people 65 years of age and older, for people from 18-64 with comorbidities for severe COVID-19, and for those whose occupational exposure puts them at high risk of COVID-19. 

For the J&J vaccine, they recommend a second dose of vaccine for all people over age 18 who received the initial dose. People who had any complications from the initial J&J vaccination such as venous thrombosis should get a second dose of an mRNA vaccine rather than a second dose of the J&J vaccine. 

A: The FDA recommends getting a booster at least 6 months after completion of the initial series of the Moderna or Pfizer vaccines. The J&J booster is recommended at least 2 months after the initial J&J vaccine.

A: Boosters are important for 2 main reasons. (1) The Delta variant is more infectious and therefore vaccinated people are becoming infected. (2) The booster dose increases the antibodies and T cells that the body makes as a result of vaccination. The levels are higher than after the initial vaccination. These responses are what protect people from infection and disease. 

A: No. The US government is using the booster doses from our national stockpile. The doses have been bought already and these doses are restricted by contract for use in the US. The US has contracted out with Pfizer, Moderna and J&J to buy doses for donation to other countries. The US government has donated more than 1 billion doses of vaccines so far.

A: The best data on efficacy and safety are from using the same booster as the vaccine you received. There are small studies in which other manufacturers’ vaccines have been used as a boost: the “mix and match” studies. Those data show the antibody responses are increased by mix and matching.  So, the FDA has authorized mix and match booster doses. This means that your booster vaccine does not have to be made by the same company as your initial vaccine(s). 

A: As long as the vaccine has been approved by the FDA, it is safe. This includes the vaccines from Pfizer, Moderna and J&J. 

A: Right now, there is no evidence that a “mix and match” booster dose will provide more benefit than getting a booster that is the same as the initial vaccination.

A: The half dose still enhances the levels of T cells and antibodies to much higher levels than after the 2 initial doses at full dosage. Several other vaccines have boosted well with “fractional dosing.” This is not a new concept in vaccinology.

A: We don’t know how long it will last. Much of this relates to how much the virus changes. We need to continue to study people’s responses to see how long they last.

A: Your side effects will most likely be like those you had when you got your original vaccination. There is no evidence so far that the side effects will be worse with the boosters. 


A: Booster doses are done in the same system as the initial vaccinations. 

A: You should bring your CDC COVID-19 Vaccination Record card when you get your booster, and it will be documented there. If you did not receive a CDC COVID-19 Vaccination Record card at your first appointment, contact the vaccination site where you got your first shot or your state health department to find out how you can get a card.  Learn more about what to do if you need a copy of your CDC COVID-19 Vaccination Record card.

A: Yes.

A: No. COVID-19 vaccines are working well to prevent severe illness, hospitalization, and death, even against the Delta variant. However, public health experts are starting to see reduced protection against mild and moderate disease, especially among certain populations.

A: Additional groups of people may be recommended to receive a booster shot as more data become available.

A: The booster vaccines are not different from the original vaccinations. They have all shown protection against the variants, including Delta. 

A: Booster doses of all three EUA vaccines (Moderna, J&J and Pfizer) increase immune responses and provide additional efficacy compared to not getting a booster. 

Vaccine FAQs

A: It is true that vaccines often have side effects, but those are typically temporary (like a sore arm, low fever, muscle aches and pains) and go away after a day or two. Many studies have proven that there is no link between vaccines and autism.

Visit the Centers for Disease Control and Prevention for more answers to questions about Vaccine Safety such as:

  • Is it safe for me to get the vaccine if I am pregnant or breast feeding?
  • Is it safe if I have an underlying medical condition?
  • Is there a risk of allergic reaction?

A: It is true that the search for vaccines and monoclonal antibodies against COVID-19 has been happening very quickly, but we’re not cutting any corners or skipping any steps. Read COVID-19 Vaccines: How Did We Get Them So Fast? to learn more.

A: Usually, the 2nd dose of an mRNA vaccine will cause more noticeable side effects, but everyone’s experience is different.3

A: Rest, and in some cases, over-the-counter medication (examples: acetaminophen, ibuprofen) might help if you have a fever or aches and pains. These medicines should not be used before getting a vaccine, only afterward to treat side effects.3

A: The risk of allergic reaction is extremely low. Talk to your doctor if you have a history of allergic reactions or anaphylaxis before getting a COVID-19 vaccine.

There are guidelines in place that may require you to be observed for more than 15 minutes after vaccination in the event of a reaction so that it can be immediately treated.7

Relevant statistics

  • Pfizer: 21 cases of allergic reaction with 2 million doses delivered (0.001%). In 15 of those 21 cases, the reaction occurred within 15 minutes.8
  • Moderna: 10 cases of allergic reaction with 4 million doses delivered (0.0003%). In 9 of those 10 cases, the reaction occurred within 15 minutes.9

A: No, it is not possible for the mRNA vaccines to impact a person’s DNA in any way. mRNA is a piece of genetic code that tells the muscle cells to make the spike protein from SARS-CoV-2 and display it for the immune system to see. You can think of this like a recipe for making food, which has the step by step directions. The vaccine doesn’t have any real ingredients that could cause infection, just the instructions. The vaccine goes to work in the outer part of muscle cells, and does not cross into the nucleus, where people’s DNA is located. You may also know that the mRNA vaccines need to be stored at very cold temperatures to keep them stable. This is because when they heat up, the mRNA starts to fall apart. Once the vaccine is given to a person, it starts to heat up in the body, and it dissolves within 1-3 days.

A: Yes. People who want to get pregnant in the future may receive the COVID-19 vaccine.

Based on current knowledge, experts believe that COVID-19 vaccines are unlikely to pose a risk to a person trying to become pregnant in the short or long term. Scientists study every vaccine carefully for side effects immediately and for years afterward, and people who get vaccinated track their symptoms. The COVID-19 vaccines are being studied carefully now, and the side effects data will continue to be studied for many years, similar to other vaccines.

There is currently no evidence that antibodies formed from COVID-19 vaccination cause any problems with pregnancy. In addition, there is no evidence suggesting that fertility problems are a side effect of ANY vaccine. People who are trying to become pregnant now or who plan to try in the future may receive the COVID-19 vaccine when it becomes available to them. 2

A: The Centers for Disease Control and Prevention has answers to more questions you may have. Find answers on the CDC website:

Pfizer Vaccine for Children Aged 5-11 Years FAQs

A: 10 microgram dose, 1/3 of the adult dose.

A: Studies looking at a range of doses in this age group were done last spring.  The studies looked at both side effects and immune responses. 

A: The most common are local side effects including pain, swelling, and tenderness on the arm where the vaccine was given.  Fever and other side effects like headache, fatigue and nausea can occur but are less common in children than adults. Less than 7% of children had a fever. Side effects were more common after the second dose.   

The CDC Vsafe and VAERS programs closely monitor reported side effects in children and adults.  There are several other studies being conducted in the United States that are looking at side effects. All of the available data indicate that the benefits of vaccination greatly outweigh the risks, including the risks associated with children having COVID-19.

A: We have not seen myocarditis following vaccines in 5–11-year-old children, but data are only available for approximately 3,100 children. Myocarditis following COVID-19 vaccines is most common in people assigned male at birth, ages 16 to 25, and has been seen very rarely in children ages 12 to 15, including boys.  The CDC reports that the risk for myocarditis in teenagers (12-15 year olds) assigned male at birth after the 2nd dose of vaccine is estimated to be 20-60 cases per 1 million doses (approximately .002% to .006% of vaccinated children).

The risk of heart problems including myocarditis in children admitted to the hospital with COVID-19 is much higher (about 2-10% of admitted children) and generally much more serious than the risk of myocarditis after the mRNA COVID-19 vaccine.

A: No. There is no evidence of this in animals or adults either.

A: No. The mRNA vaccine does not enter the cell nucleus, which is where your DNA is located. The vaccine is only able to deliver the instructions that teach your body how to recognize the virus, and it breaks down naturally within a few days after vaccination.

A: 4500 children ages 5-11 years participated in the study.

A: Children were in the study for 2-5 months, which started in June 2021. 

A: The data from studies in children ages 5-11 showed that this dosage is safe and effective. This is reinforced by the strong safety profile of the Pfizer vaccine in adolescents, teenagers, and adults, with more than 400 million doses given.  Children ages 5-11 made similar antibody responses as adults do. The children participating in the Pfizer study will continue in the study for a total of two years so that their safety can continue to be monitored, and updates will be shared as they become available.  

A: We have good data on thousands of children who have participated in the research studies, although it is true that follow-up in children has not been for as much time as adults.  This follows the usual vaccine process.  After you establish safety and efficacy in adults, you then move to teenagers and young adults, then older children, followed by younger children and infants.

Children ages 5-11 can get sick with COVID-19. Although most won’t be hospitalized or need the Intensive Care Unit (ICU), right now COVID-19 is the 8th leading cause of death in children. Sometimes they can get really sick. Children with COVID-19 are hospitalized all the time and require oxygen or breathing machines or even heart-lung support. This disease is preventable with vaccines and it’s important to know that thousands of children have gotten very sick already in the US alone. There is no way to predict who will get sick with more serious symptoms, although we do know that some children who have other medical conditions or who are overweight tend to get sicker.

In addition, children can spread the virus to their families and especially people like their grandparents, who are at even higher risk of serious disease or death due to COVID-19.  Right now, with the Delta variant of virus going around that is very contagious, it is much better to get your child immunized now rather than waiting.

A: We don’t know yet. This can only be learned by the ongoing participation of children in the research studies. Studies in adults have already shown that the immune responses, like making antibodies, that people have from the vaccines last longer than the natural antibodies people make from having COVID-19. As new data is learned from the research, we will continue to make it available.

A: No.  Immunity from SARS-CoV-2 infection in children is not long lasting.  Immunity from SARS-CoV-2 infection appears to be related to how sick one gets, and children generally have less severe disease and therefore have less antibodies to protect them.   Antibodies made in response to the vaccine are much higher and block the virus so that it cannot infect people’s cells.  In studies at Seattle Children’s Hospital, antibodies after having COVID-19 go down pretty quickly over the first 6 months after infection. With this in mind, and knowing the safety data from the research, it is safer for children to get vaccinated rather than waiting to see if they will get COVID-19.  

Among children who tested positive for SARS-CoV-2 infection and were then vaccinated, their antibody levels were higher after vaccination. Because of this, it is also recommended that children who have already had COVID-19 should be vaccinated as well.

COVID-19 Clinical Studies FAQs

A: There are over 7 billion people in the world. COVID-19 is affecting everyone, and vaccines are needed to end the pandemic. Now that we have vaccines that have proven to be very effective at preventing severe COVID-19 disease, why do we need to continue COVID-19 vaccine research?

There are three main reasons:

  1. If we find several vaccines that work, we can end the pandemic sooner.
  2. In addition to meeting the demand, we also need vaccines that can work for many kinds of people around the world.
  3. We still need more data over a longer period of time to answer key questions.

Read more about continuing research after some vaccines have been approved by the FDA.

A: No. That type of study design is known as a challenge study. Instead, we are using a design known as “randomized, blinded, placebo-controlled” studies. We will enroll people who are more likely to be exposed to the virus and develop COVID-19 illness. Some participants will get the vaccine, and some will get a placebo, which is a sterile saline solution that does not have any vaccine in it. We expect that some people will be exposed to the virus in their everyday lives, and may become sick. We will compare the 2 groups to see if there are fewer people who get sick in the vaccine group than in the placebo group. This is how we will know if the vaccine works.

A: Unlike guinea pigs, people can say yes or no about joining a study. All study volunteers must go through a process called informed consent that ensures they understand all of the risks and benefits of being in a study, and those volunteers are reminded that they may leave a study at any time without losing any of their rights or benefits. The COVID-19 Prevention Network (CoVPN) takes great care to make sure people understand the study fully before they decide whether or not to join. All CoVPN studies follow US federal regulations on research, as well as international ethical standards and any country-specific requirements for the countries where our research is conducted.

A: In order to find a vaccine or antibody that works in all kinds of people, it is necessary to test them in all kinds of people. This is especially true for groups of people that have been hardest hit by the COVID-19 pandemic. Protecting the well-being of study volunteers is the greatest responsibility in every study, and the CoVPN works to make sure that our studies follow the highest ethical standards. These studies are done in collaboration with local scientists and researchers and community representatives with oversight by ethics and regulatory groups in each country. Many studies are done in sub-Saharan Africa, Europe, North America, and South America at the same time and we follow the same procedures and international standards no matter where the study takes place.

A: No, there is no way to guarantee whether you will get the vaccine or the placebo. All the studies will be blinded, which means that neither you nor the clinic staff will know who is getting which product. When someone decides to enroll, it will be a computer who makes a random assignment to the study product group or the placebo group, and only the pharmacist will know which product you get. These steps are necessary to ensure that no assumptions are made, and any immune responses or side effects are dealt with equitably, without favoritism.

A: Yes. All study participants are encouraged to get an approved vaccine if one is available to them. You are asked to talk to the study staff so that they can follow the process to tell you if you got the vaccine or placebo, and if there are any safety or timing considerations for you to follow. Being in the study is not a barrier to getting an approved vaccine.

A: If the vaccine is found to be effective and is granted Emergency Use Authorization (EUA) by the FDA, it will be made available to the people in the placebo group of the study. This is described in more detail in each study’s informed consent form.

A: The trials are enrolling people with a variety of health conditions, because we know that having underlying health conditions is one of the factors that puts people at risk for more severe COVID-19 illness. Each study is different. If you complete the survey to join our Volunteer Screening Registry, a local study team would be able to follow up if they feel you are a good match to a study they have available. Once they reach out to you, you can have a detailed conversation to discuss your concerns and health conditions.

A: We have clinics all over the U.S., but we’re not able to provide a list of specific locations at this time, in part because site selection is still being finalized for each study. Different clinics will be running each of the different studies that are coming up in the next several months. Some trials will also be done internationally. Additional sites are often added to a study to follow where COVID-19 outbreaks are happening or to better serve priority populations, and some sites may close early if they have enrolled their allotted number of participants. The list of clinics that are participating can change frequently.

If you complete the survey on our website, your information will go into our Volunteer Screening Registry. This will enable a local clinic near you to follow-up with you if they feel you would be a good match for a study they have available. This contact could happen soon, or it may be in several months, as there are several studies taking place over the next 6 months. If you do not get contacted, it could mean that there is no participating clinic near you, or that you are not a good match to the eligibility criteria of the study the clinic has available.

A: Yes, people who join a study get compensated for their time and inconvenience. The amount per visit varies depending on how long the visit is and the procedures that take place. The amount also varies from city to city, because the cost of living is different between large metropolitan areas compared to smaller towns and rural areas. The details about compensation will be explained when a person goes through the informed consent process at a local clinic to join a study.

A: When you complete the submission of your survey, you will see a message with the heading “Success!” which means that we have successfully received your information. Our system does not send out an email confirmation message. Your information will be provided to a study site closest to your location. If the study site thinks you may qualify for a particular study, they will reach out to you as soon as possible about participating. We will never ask for your financial, credit, or bank account information. We will never ask for an ID number over the phone.

A: There are many clinical trials evaluating therapies and vaccine candidates against severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), as well as studies of people who have recovered from infection. To find out more about those clinical trials, please visit NIAID’s Volunteer for COVID-19 Clinical Trials page or


A novel coronavirus is a new coronavirus that has not been previously identified in people. Coronavirus disease 2019 (COVID-19) is caused by the new virus called severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). This is not the same as the coronaviruses that commonly circulate among humans and cause mild illness, like the common cold.

A: COVID-19 can affect anyone, regardless of their race or ethnicity. However, data show that it is disproportionately impacting Black/African American, Latinx, and American Indian/Alaska Native/Native Hawaiian populations. This is largely due to social and structural factors that impact some communities more than others. These factors include poverty, poor housing conditions, challenges with accessing medical care, and limited resources, among others.

A: No. Antibiotics help stop bacterial infections and are not effective against viruses such as SARS-CoV-2.1

A: No. Some people who do get SARS-CoV-2, the virus that causes COVID-19, experience flu-like symptoms. COVID-19 currently has a higher death rate than the flu and appears to be more contagious than the flu.2 We do not know yet whether COVID-19 will change with the seasons like influenza. In the US, the CDC recommends that everyone aged 6 months and over receive an influenza vaccine every year.

A: Yes. Scientists have found that people who have SARS-CoV-2 and have no symptoms (i.e., asymptomatic) can still pass along SARS-CoV-2 to others.2 In fact, people infected with SARS-CoV-2 seem to be most infectious before they even show signs and symptoms.

A: Depending on your age, health history, and how long you have had COVID-19 symptoms, you may qualify for treatment with a monoclonal antibody. Treatment with a monoclonal antibody can reduce the amount of the SARS-CoV-2 in the body. Having a lower amount of virus in the body means you may have milder COVID-19 symptoms and decrease the likelihood of being hospitalized or dying.

Monoclonal antibody treatment may help people who:

  • Have mild to moderate symptoms of COVID-19, and
  • Have tested positive for COVID-19 sometime in the last 10 days, and
  • Are at high risk of getting more serious symptoms.

Currently, the following monoclonal antibodies have been authorized for emergency treatment use by the FDA for people who are 12 years or older, who weigh at least 88 pounds, and are at high risk for severe COVID-19:

Monoclonal antibody treatments for COVID-19 may only be administered by licensed medical providers. Please speak with your medical provider or contact your local or state public health department for more information.

Learn more about monoclonal antibodies.

Other drugs that may be given to people who are hospitalized with COVID-19 include:

  • Dexamethasone, an anti-inflammatory drug that has been shown to be beneficial for people on ventilators.
  • Remdesivir, a drug that speeds up recovery time for people hospitalized with COVID-19.

A: Face masks can reduce your chances of getting SARS-CoV-2, but the primary purpose of face masks is to protect others from you in case you are infected, especially if you are not showing symptoms. It is recommended that the public wear well-fitting multi-layer cloth masks, including homemade masks, to help prevent transmitting the virus to other people. You may also want to consider wearing 2 masks, which can offer additional protection if your mask is a little loose and doesn’t fit snugly around your nose and mouth. 4

A: SARS-CoV-2 can be transmitted in all areas, including hot/humid and cold climates.1

A: While gargling with warm saltwater can make a sore throat feel better, it has no effect on SARS-CoV-2.5

A: There are simple things you can do to help keep yourself and others healthy. Wash your hands often with soap and water for at least 20 seconds, especially after blowing your nose, coughing, sneezing, going to the bathroom, and before eating or preparing food. When in public, wear a well-fitting multi-layer cloth face mask that covers your mouth and nose. You may also want to consider wearing 2 masks, which can offer additional protection if your mask is a little loose and doesn’t fit snugly around your nose and mouth. Avoid touching your eyes, nose, and mouth with unwashed hands. Stay home when you are sick. Cover your cough or sneeze with a tissue, then throw the tissue in the trash.2 Stay 6 feet apart from other people when you are out in public.

A: No. You can help stop COVID-19 by knowing the signs and symptoms, which can include:

  • Fever
  • Cough
  • Shortness of breath
  • Chills
  • Sore throat
  • New loss of taste and/or smell

Seek medical attention immediately if you or someone you have been exposed to has emergency warning signs, including:

  • Trouble breathing
  • Persistent pain or pressure in the chest
  • New confusion or not able to be woken up (from sleep)
  • Bluish lips or face

This list of symptoms is NOT all inclusive and is being updated as new information becomes available. Please consult your medical provider for any other symptoms that are severe or concerning.2

General FAQs

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A: We’re here to help. If you still have questions after reading the above FAQs, please contact us at, or call (206) 667-2001. Due to the large volume of inquiries, our response may be significantly delayed. We will prioritize responding to questions that are not addressed in these FAQs. Please note:

  • The COVID-19 Prevention Network does not provide personal medical advice or diagnoses.
  • If you live in the U.S. and are experiencing a medical emergency, call 911 and notify the operator if you think you might have COVID-19.
  • If you think you have been exposed to COVID-19 and develop a fever and symptoms such as a cough or difficulty breathing, call your healthcare provider immediately for medical advice.


  1. World Health Organization, Coronavirus advice for the public: Myth busters
  2. Centers for Disease Control and Prevention (CDC), Coronavirus COVID-19
  3. Centers for Disease Control and Prevention (CDC), Key Facts
  4. U.S. Food and Drug Administration, Coronavirus (COVID-19) Update: FDA Continues to Facilitate Development of Treatments
  5. Johns Hopkins Bloomberg School of Public Health, COVID-19 Myths vs. Realities
  6. Johns Hopkins Medicine, Coronavirus Disease 2019: Myth vs. Fact
  7. Centers for Disease Control and Preventions: What to Do if You Have an Allergic Reaction After Getting A COVID-19 Vaccine
  8. Centers for Disease Control and Preventions: Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Moderna COVID-19 Vaccine — United States, December 21, 2020–January 10, 2021
  9. Centers for Disease Control and Preventions: Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Pfizer-BioNTech COVID-19 Vaccine — United States, December 14–23, 2020

Together we can help end uncertainty. Join us.

We need your help to keep learning and improving vaccines against COVID-19. We made history with safe and effective vaccines and we’ll keep working as long as there are people to protect, variants emerging, and research answers needed to end this pandemic. To participate:

  • Fill out the survey again so we have your most current information to match you with a study, even if you filled it out previously!
  • If you haven’t registered to volunteer before, sign up today. There’s still a lot we can do together for those we love.


Content last reviewed on December 2, 2021