Frequently Asked Questions

On August 23, the U.S. Food and Drug Administration granted full approval to the Pfizer-BioNTech COVID-19 vaccine for individuals 16 and older. To date, two other COVID-19 vaccines, one produced by Moderna and another by Johnson & Johnson, have received FDA emergency approval in the United States.  The Pfizer vaccine is also available to children 5 to 15 years old under emergency use authorization. While all of these vaccines were shown to be safe and effective through clinical trials, it’s natural to have questions about them.

You can check this page for updates and science-based answers to frequently asked vaccine questions.

How do I know the COVID-19 vaccines are safe?

All approved COVID-19 vaccines (Johnson & Johnson, Moderna, Pfizer) went through all the required stages of clinical testing needed to prove two things: that they’re safe and work to protect against COVID-19.

Real-world data backs this up. About 420 million vaccine doses have been administered in the United States so far and more than 7 billion doses have been administered around the world. Very few serious reactions have been reported.

Four conditions have occurred in vaccinated people. Some individuals developed anaphylaxis (a strong allergic reaction). Others developed myocarditis (inflammation of the heart muscle). Some people who received the Johnson & Johnson vaccine have reported a certain type of blood clot called thrombosis with thrombocytopenia syndrome (TTS) or Guillain Barré Syndrome, a condition in which the immune system attacks the nerves. Of the millions of people who were vaccinated, only a handful have developed any of these conditions.

Out of every one million vaccinated people between two and five people went into anaphylaxis. The CDC is still monitoring cases of myocarditis (you can find the latest numbers here), but about 1,000 cases have been confirmed. Of the 15.5 million J&J vaccines administered, 48 developed these blood clots as of November 3, 2021. And, there are only 244 reports of Guillain Barré in the CDC’s vaccine safety monitoring system as of October 27 (the CDC regularly updates numbers here).

The risk of experiencing these complications is extremely low. Meanwhile, there are serious risks associated with COVID-19, including severe illness or hospitalization.

The CDC takes safety very seriously and continues to monitor these vaccines calling the effort “the most intense safety monitoring” in U.S. history.

Do vaccines protect people in the real world?

Yes! One especially notable study was released in September 2021 by the CDC. The study followed 3,689 people between March and August 2021. The results showed that the Moderna vaccine was 93 percent effective against hospitalization, the Pfizer vaccine was 88 percent effective against hospitalization and the J&J vaccine was 71 percent effective against hospitalization. This study evaluated the efficacy against the original coronavirus variant rather than the Delta variant. There are other studies in the U.S., Israel and Scotland that all speak to the effectiveness of these vaccines in the real world.

Scientists are still studying how long immunity lasts and how different variants may affect it. There is evidence showing that the vaccines can protect against the Delta variant. For example, one CDC study analyzed the vaccines’ ability to prevent visits to the emergency room, or urgent care clinics in nine states, between June and August 2021 when the Delta variant was surging in the U.S. The study found that all three approved vaccines were collectively 86 percent effective in preventing hospitalization. When broken down by vaccine type, Moderna, Pfizer and J&J were respectively 95 percent, 80 percent and 60 percent effective against hospitalization in this study. This study is the largest U.S.-based real-world study of vaccine effectiveness to date and it suggests that the vaccines continue to be protective against the Delta variant.

Do I need a booster shot if I am already vaccinated?

The original coronavirus vaccines still protect against severe illness and death, but the CDC has now authorized booster shots for all adults because there is some evidence that vaccinated people can still get “breakthrough infections,” and that the strength of vaccines can wane over time. 

Studies have shown that booster shots are safe, and can help protect you. For example, a study in Israel of over one million people found that booster shots were 92 percent effective in preventing severe COVID-19. And, according to one pre-print study released in November, serious side effects were reported in less than .01 percent in a sample of about 48,000 boosted people. 

If I Received the Pfizer or Moderna Vaccine, Am I Eligible For a Booster Shot?

If you are older than 18 and received the second Pfizer or Moderna shot 6 or more months ago, you can now receive a booster shot according to the latest CDC guidance. You can receive a booster shot made by J&J, Moderna or Pfizer, regardless of which shot you had already received.

If I Received the J&J Vaccine, Am I Eligible For a Booster Shot?

If you are 18 and older and received the J&J shot two or more months ago, you are eligible to receive a booster shot made by J&J, Moderna or Pfizer.

Can You Mix and Match Booster Shots?

You may receive a booster shot made by any manufacturer.

For example: if you first received a Moderna vaccine and are eligible for a booster, you can receive a booster shot made by Pfizer or J&J. You would receive that booster at least six months after getting your second dose.

Here’s another example: If you received a J&J vaccine and are eligible for a booster, you can receive one made by either Pfizer or Moderna. You would receive your booster two months after your first shot.

The National Institutes of Health has been studying the safety and efficacy of mixing and matching booster shots since June. Preliminary results have been released. The study evaluated antibody responses (they’re a way to measure how your immune system responds to an invader) of 458 people who received a booster shot. The boosters raised people’s antibody levels regardless of which booster shot they received.

Is It Safe to Mix and Match Booster Shots?

The National Institutes of Health has been evaluating the safety and efficacy of booster shots since June. The preliminary study evaluated antibody responses of 458 people who got a booster shot that was different from the vaccine they originally received.

No serious side effects related to booster shots were reported in the study – no matter what type of booster shot was given. Instead, side effects tended to be mild and similar to what people experienced during their first round of shots. Over half of participants reported injection site pain, malaise, headaches or muscle aches.

Other countries have also used the mix and match strategy safely. In the U.K., one study evaluated the safety of mixing the AstraZeneca vaccine (not approved in the U.S.), with the Pfizer vaccine. There were no severe events reported, though people who had mixed and matched vaccines had slightly more frequent mild or moderate side effects. These studies are still limited, but so far, data suggests that mixing and matching vaccines has not caused harm and could increase the immune system’s ability to fight COVID-19.

What about the variants? Does the vaccine protect against them too?

Research is still ongoing into how vaccines fare against certain variants, but there are signs that they still offer protection.

Protection Against Delta

The Delta variant now makes up 93 percent of COVID-19 cases in the U.S, per NYU Langone. Vaccines are still effective in preventing severe disease and death from the Delta variant.

Real-world data backs this up. An August CDC study on healthcare workers found that vaccine efficacy declined from 91 to 66 percent as Delta surged, but this is still a very high level of protection. Even more recent research, published in September suggests protection is even higher than that, though.

All three vaccines were 86 percent effective in preventing hospitalizations. However, protection for older people was slightly lower. The vaccines were 76 percent effective for people 75 and older. This is still a very high level of protection, suggesting that in the face of Delta, these vaccines still work.

A new CDC study looking at COVID-19 cases, hospitalizations and deaths in 13 states between April and July showed that unvaccinated people were 10 times more likely to be hospitalized for COVID-19 and 11 times more likely to die of it than people who got the vaccine. This is strong evidence showing that, even in the face of Delta, the vaccines are doing their job.

Protection Against Lambda

The World Health Organization currently designates Lambda as a “variant of interest,” but it is not yet designated as a “variant of concern.” In short, scientists are keeping an eye on it.

There have been fewer than 1,000 Lambda cases detected in the U.S., as of September 13. The primary variant of concern at the moment is the Delta variant.

A pre-print (not peer-reviewed) study suggests antibodies produced by Pfizer and Moderna vaccines and to a lesser extent by J&J could neutralize the Lambda variant.

Protection Against Mu

As of September, the Mu variant is also listed as a variant of interest according to the World Health Organization. But it is not yet classified as a variant of concern.

The CDC has not listed Mu as a variant of interest yet. In September Anthony Fauci, the Director of the National Institute of Allergy and Infectious Diseases said the agency was paying attention to Mu, but it is not considered an immediate threat.

Should I get a vaccine if I’ve already had COVID-19?

The short answer is yes.

While you may have COVID-19 antibodies (proteins that bind to the virus and tell the body to vanquish it) in your body after you recover, we don’t know how long immunity gained from natural infection lasts.

Reinfection with COVID-19 is rare but CDC research has shown vaccinated people are less likely to be reinfected than unvaccinated people. One study examined 246 Kentucky residents who got COVID-19 once in 2020 and 492 people who had tested positive for the virus twice (the second time they tested positive in May-June of 2021). The unvaccinated people were 2.3 times more likely to have gotten the disease again, compared to vaccinated people.

Other studies suggest as many as one-third of mild COVID-19 cases don’t leave behind protective antibodies. One CDC study from September 2021 followed 72 people, most of whom had mild cases. About 36 percent of them did not develop antibodies.

It’s possible more severe cases of COVID-19 may leave behind more antibodies, but these cases are very risky. Even if the case doesn’t lead to hospitalization or death, there could be long-term consequences from the condition.

The most reliable way to train your immune system to fight COVID-19 is to get vaccinated, rather than risking a so-called “natural infection.” You don’t need to risk short-term or long-term consequences of the virus to protect yourself.

There’s one more reason to get vaccinated: You can help protect others. One study on 365,000 households in England, for instance, found that when some family members were vaccinated, they were half as likely to pass the original coronavirus to an unvaccinated (and vulnerable) person.

The Delta variant is even more contagious than the original coronavirus, which means that vaccinated people can also spread that version of the virus. However, the vaccine will still protect you from getting very sick from Delta. Reducing the chances of getting very sick can also help others by keeping the hospitals from becoming overwhelmed.

If I already got one Pfizer/Moderna shot, why do I need to get the second one?

Real-world research has shown that immunity is improved after two shots. For instance, the effectiveness of one Pfizer or Moderna vaccine against the original coronavirus was 64 percent in a study of U.S. healthcare workers done by the CDC. But two shots raised the efficacy to around 94 percent in that study.

In light of new variants such as Delta and Lambda, it’s even more important to get both doses of your COVID-19 shot.

For example, a Public Health England September report shows that one dose of a vaccine is just 35 percent effective against the Delta variant. Whereas, two doses of the vaccines were 79 percent effective against the Delta variant.

Should I get the vaccine if I’m pregnant or want to become pregnant?

You should talk to your doctor if you are pregnant and are worried about getting a COVID-19 vaccine. However, the CDC now strongly recommends pregnant people (or those who want to become pregnant) get the COVID-19 vaccine.

One study published in the New England Journal of Medicine followed over 35,000 pregnant women who got vaccinated. The study found no signs that the vaccine was linked to any complications. A pre-print study released in August shows that mRNA vaccines were also not linked to any increased risk of miscarriage.

Meanwhile, evidence also shows that pregnant women who get COVID-19 are at higher risk of getting very sick from the virus itself compared to non-pregnant women of the same age. This means protecting against the virus with a vaccine is even more important.

Does the vaccine affect fertility?

You may have heard some misinformation recently suggesting that the vaccines might impair a woman’s ability to get pregnant. Scientists have debunked this claim.

In a joint statement, the American Society for Reproductive Medicine, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine said there is no evidence the vaccine impacts women’s fertility. The idea that there could be fertility loss due to vaccination was “scientifically unlikely,” the statement read.

For men, a study done at the University of Miami found no evidence that men who got Pfizer or Moderna vaccines had any irregularities in their sperm as a result of vaccination.

The Mayo Clinic also notes that there is “no evidence” that COVID-19 vaccines can cause fertility problems.

Are children eligible for the vaccine?

Children aged five and older are eligible for Pfizer’s two-dose coronavirus vaccine. If your child is 12 and older, they will receive the same version of the vaccine given to adults. If they are between five and 11, they receive a slightly lower dosage.

How safe is the vaccine for children?

The CDC currently recommends that children aged five years or older get the Pfizer vaccine. These recommendations were made after studies specifically evaluated the safety of this vaccine in children and the data was reviewed by outside experts.

For example, a clinical trial on 2,260 adolescents aged 12 to 15 found no serious adverse effects linked to the Pfizer vaccine. Meanwhile, the vaccine was 100 percent effective in protecting against COVID-19.

In one ongoing study on about 3,100 children aged 5-11, no serious side effects have been detected, according to the FDA. So far, studies done by Pfizer suggest the vaccine was 90.7 percent effective against COVID-19 infection, including infection with the Delta variant.

One factor scientists weighed when approving these vaccines was the risk of myocarditis, a type of inflammation present in the heart muscle. Myocarditis was the most common serious side effect in one August 2021 study of 8.9 million adolescents, however, there were only 397 cases detected in total. It’s most common in males aged 16-29, per CDC research.


Experts suggest myocarditis risk is lower for children younger than 15. Additionally, COVID-19 is more likely to lead to myocarditis than vaccination.  A CDC study released in August found that children under 16 who got COVID-19 have 37 times the risk of developing myocarditis than those who did not get the virus. The vaccine available for children over five will protect against most COVID-19 infections.

How effective is the vaccine for kids?

Children five and older are eligible to receive Pfizer’s coronavirus vaccine. Real-world data has shown that vaccines have already been protecting adolescents, who became eligible to receive their shots earlier than children younger than 12.

For example, Israel made the Pfizer vaccine available to adolescents in early June. By July, the country was dealing with an outbreak of the Delta variant – now the predominant variant in the U.S. as well. A study conducted in July in Israel found that, in adolescents aged 12-15, the vaccine was about 91.5 percent effective against infection with Delta. That efficacy was achieved between two and four weeks after receiving the second dose.

Scientists are still collecting data to determine how long this protection lasts. But this study demonstrates that the vaccine provides adolescents with strong, short-term protection against infection with the Delta variant.

You might have heard of terms like “breakthrough infections” or have noted some places have reinstated mask mandates.

While these changes can seem scary, the bottom line is vaccines are still the best tool we have to protect ourselves and our families from COVID-19. Masks are an additional tool to help protect people who haven’t yet been vaccinated or lower the chances of spreading the virus to another person.

Even in a world with safe and effective vaccines, public health researchers are constantly researching SARS-CoV-2 and other coronavirus variants. This work will inform the other safety precautions we may need to take, which means that guidelines can change as scientists learn more. Here is what we know so far about breakthrough infections and masks.

What is a breakthrough case?

A “breakthrough case” occurs when a person tests positive for COVID-19 at least 14 days after they have been fully vaccinated. You are deemed fully vaccinated after receiving two doses of Moderna and Pfizer vaccines and one dose of Johnson and Johnson’s vaccine.

You may have noticed an uptick in news coverage about breakthrough cases lately. It isn’t a reason to fear that the vaccines aren’t working. Since so many people are being vaccinated, the CDC expects to see some breakthrough infections occur.

Initial studies of the vaccines indicated that Moderna and Pfizer’s vaccines were about 95 percent effective in preventing disease and Johnson and Johnson’s were about 66.3 percent effective. Even in these cases, there is some chance that you could get sick after being vaccinated. But your chances of getting sick without a vaccine remain far higher.

Even as we battle the more contagious Delta variant, evidence suggests these vaccines are still protecting most people. One CDC study conducted on about 600,000 COVID-19 cases across 13 states showed unvaccinated people were 4.5 times more likely to get a COVID-19 infection, 10 times more likely to be hospitalized and 11 times more likely to die of COVID-19 compared to vaccinated people.

Even if a breakthrough infection occurs, there is a lower chance of getting extremely ill from it and the vaccine still protects most people from getting sick at all.

How common are breakthrough cases?

Due to incomplete data, it’s impossible to know for sure how common breakthrough cases are. The CDC doesn’t track breakthrough cases that don’t result in hospitalization or death, as of May 2021.

Right now, data suggests they’re rare. In October, one pre-print (not yet peer-reviewed) study from North Carolina found that about 1.9 percent of 16,020 participants reported a breakthrough infection after vaccination. People who received J&J vaccines or were younger than 45 reported slightly more breakthrough infections in the study. This study has limitations, but it suggests that breakthrough cases are still very rare.

In July 2021, data collected from 25 states by the Kaiser family foundation showed that breakthrough infections only occurred in less than one percent of vaccinated people in all of those states. The analysis estimates that the rate of breakthrough cases ranged from .1 percent to .29 percent.

Are breakthrough cases dangerous?

The majority of the individuals who got COVID-19 after being fully vaccinated, experienced less severe symptoms, the CDC notes.

Even in the case of the Delta variant, it seems breakthrough cases are rarely severe. In New York, for example, .07 percent of fully vaccinated people have been hospitalized. In Minnesota, there’s a similar pattern: .08 percent of fully vaccinated people have been hospitalized.

Ultimately, the risks of getting very sick from COVID-19 are far higher if you are not vaccinated. The COVID-19 hospitalization rate is 12 times higher in unvaccinated people, compared to vaccinated people, per the CDC.

However, it is still important to stay home if you have any symptoms of COVID-19. While the risks of getting very sick are low, if you are infected with the Delta variant, you could risk passing on a case to someone else.

Do I need to wear a mask if I am vaccinated?

As of July 27, 2021, yes. The CDC now recommends fully vaccinated people wear masks indoors if they live in areas of the country where the virus is surging.

It is a reversal from the CDC’s May guidance due to new information about the Delta variant and its transmissibility.

Early-stage studies have shown that people infected with the Delta variant carry far more virus (called “viral load”) than those infected with the original coronavirus. Having a higher viral load doesn’t make people get sicker, but it is a factor that makes it easier to pass the virus to others.

Other preliminary studies, though, suggest viral load declines faster in vaccinated people compared to unvaccinated people, which could impact our ability to continuously spread it.

While vaccines still protect against severe illness, they may not be as effective in preventing someone from passing the Delta variant on to another person. That’s why wearing masks is still important.

Studies show masks can stop us from passing on the coronavirus – one study in Beijing found that masks were 79 percent effective in preventing transmission within a home, for example. Masking in high transmission areas or enclosed spaces can help stop us from spreading the disease to vulnerable people.

Do kids need to wear masks at school?

The CDC still advises universal indoor masking, though, as more children are vaccinated, some schools are loosening mask mandates. Check with your local schools for the most up-to-date information in your area. 

Wearing masks in school helps protect immunocompromised students or students too young to be vaccinated. In light of information on the contagiousness of the Delta variant, masks will also help slow the spread of COVID-19.

The information on this site is not intended or implied to be a substitute for professional medical advice, diagnosis or treatment. All content, including text, graphics, images and information, contained on or available through this website is for general information purposes only. No action or inaction should be taken based solely on the contents of this information; instead, readers should consult appropriate health professionals on any matter relating to their health and well-being. The Cura Foundation makes no representation and assumes no responsibility for the accuracy of information contained on or available through this website, and such information is subject to change without notice. You are encouraged to confirm any information obtained from or through this website with other sources and review all information regarding any medical condition or treatment with your physician. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.

The Cura Foundation does not recommend, endorse or make any representation about the efficacy, appropriateness or suitability of any specific tests, products, procedures, treatments, services, opinions, health care providers or other information that may be contained on or available through this website. THE CURA FOUNDATION IS NOT RESPONSIBLE NOR LIABLE FOR ANY ADVICE, COURSE OF TREATMENT, DIAGNOSIS OR ANY OTHER INFORMATION, SERVICES OR PRODUCTS THAT YOU OBTAIN THROUGH THIS WEBSITE. In the event of a medical emergency, call a doctor or 911 immediately.