Frequently Asked Questions about Vaccines

On September 1, 2022, the CDC issued new recommendations for COVID-19 boosters, after the FDA authorized updated booster formulas from both Pfizer and Moderna. The CDC recommends that everyone who is eligible stay up-to-date on vaccinations by getting an updated booster dose at least 2 months after their last COVID-19 shot—either since their last booster dose, or since completing their primary series. Pfizer’s updated booster shot is recommended for individuals 12 and older, and Moderna’s updated booster shot is recommended for adults 18 and older. 

These new boosters contain an updated bivalent formula that both boosts immunity against the original coronavirus strain and also protects against the newer Omicron variants that account for most of the current cases. Updated boosters are intended to provide optimal protection against the virus and address waning vaccine effectiveness over time.

Eligible individuals can get either the Pfizer or Moderna updated booster, regardless of whether their primary series or most recent dose was with Pfizer, Moderna, Novavax, or the Johnson & Johnson vaccine.

As per the CDC’s recommendations, the new bivalent booster replaces the existing monovalent vaccine booster, therefore that vaccine will no longer be authorized for use as booster doses in people age 12 and up. 

Added September 2, 2022 

Yes, the CDC recommends that everyone age 12 and up should get an updated COVID-19 booster this fall to stay up-to-date on vaccinations. The same is true for people who completed their primary series or received one or two boosters: they should get an updated booster dose at least two months after their last shot.

For maximum effectiveness of the updated booster dose, individuals who recently had COVID-19 may consider delaying any COVID-19 vaccination, including the updated booster dose, by 3 months from the start of their symptoms or positive test.

Added September 2, 2022 

No. The updated bivalent formula is in use only for COVID-19 booster doses, and not for initial vaccination. The best way to protect yourself from getting severely ill from COVID-19 is to get vaccinated. The CDC recommends that currently unvaccinated people get their primary series (the initial two doses of either Pfizer or Moderna, or one dose of the Novavax vaccine), and then wait at least two months to get the updated Pfizer or Moderna booster dose.

Added September 2, 2022 

As of September 1, 2022, updated Pfizer booster doses are authorized for individuals age 12 and older. The CDC continues to recommend that children age 5 and up get a booster dose at least 5 months after completing their primary series. For this younger age group (children 5-12), the original booster dose formula is still authorized for use.

As scientific experts at the FDA and CDC continue to review the data, updated Omicron-specific boosters could become available for younger children in the future.

Added September 2, 2022 

Booster doses are common for many vaccines, and over time, booster doses may need to be updated to provide optimal protection against new variants of the virus. The scientists and medical experts who developed the COVID-19 vaccines continue to watch for waning immunity, how well the vaccines protect against new mutations of the virus, and how that data differ across age groups and risk factors. 

To date, booster doses have worked well in extending the protection of the vaccine against serious illness, but have been somewhat less effective in boosting immunity against new variants of COVID-19 compared to the original strain. The updated booster dose formula is designed to protect against original strains of the virus, as well as Omicron variants that account for the majority of current new infections.

The latest CDC recommendations on booster doses help to ensure more people across the U.S. are better protected against COVID-19. The best way to protect yourself from COVID-19 is to get vaccinated and boosted if eligible. Vaccination and boosting is particularly important for individuals more at risk for severe COVID-19, such as older people and those with underlying medical conditions.

Updated September 2, 2022 

Yes. Eligible individuals can get either the Pfizer or Moderna updated booster, regardless of whether their primary series or most recent dose was with Pfizer, Moderna, Novavax, or the Johnson & Johnson vaccine.

Updated September 2, 2022 

On July 19, 2022, the CDC updated its COVID-19 vaccine recommendations, approving the Novavax vaccine for emergency use authorization for adults 18 years and older. Novavax is a two-dose, protein-based COVID-19 vaccine that is currently being used in more than 40 countries and has been authorized by the European Union and the World Health Organization.

Novavax will now be the fourth COVID-19 vaccine available in the U.S., in addition to Pfizer, Moderna, and Johnson & Johnson. As a protein-based vaccine, Novavax is another option for people who are allergic to one of the components in a mRNA or viral-vector vaccine. The vaccine is currently authorized as a primary series only, and not as a booster dose.

Regulators authorized the vaccine following an extensive review of clinical trials and safety and effectiveness data. At the time of the announcement, the CDC stated that the Novavax vaccine would be available in the coming weeks.

Updated July 20, 2022 

The Novavax vaccine is created using more traditional protein-based technology for vaccine development, unlike the other vaccines currently available in the United States (the Pfizer and Moderna mRNA vaccines and viral-vector Johnson & Johnson vaccine). 

The Novavax vaccine uses a combination of purified coronavirus spike proteins and an immune-boosting stimulant called an adjuvant (common in many vaccines) to strengthen the body’s immune response against COVID-19. Novavax has already been authorized in more than 40 countries and has been granted emergency authorization from the European Union and the World Health Organization.

Updated June 29, 2022 

Data from the Novavax clinical trial also show that Novavax is more than 90% effective at protecting against symptomatic COVID-19, and 100% effective against severe illness and death. Common side effects include soreness at the injection site, fatigue, muscle pain, and headaches. 

In terms of serious adverse reactions to Novavax, data show there were six cases of myocarditis from a clinical trial of about 30,000 people, primarily among young men. The cases of myocarditis in the clinical trial were treatable, and all six individuals recovered well. The risk of developing myocarditis from COVID-19 remains higher than the risk of developing it from a COVID-19 vaccine, including Novavax.

Updated June 29, 2022 

On May 5, the FDA updated its authorization of the Johnson & Johnson COVID-19 vaccine, limiting its use to adults 18 and older in certain cases where other vaccines are not appropriate. This means that most people should receive either the Moderna or Pfizer vaccine, and should only get the Johnson & Johnson vaccine if other vaccines are not accessible or clinically appropriate (such as people who have had an anaphylactic reaction to a different COVID-19 vaccine) or in cases where individuals who elect to get a Johnson & Johnson vaccine would otherwise not receive a COVID-19 vaccine. 

This update was based on the latest data about the risk of developing a rare blood clotting disorder after receiving the Johnson & Johnson vaccine. While the risk of a serious adverse reaction to the Johnson & Johnson vaccine is very low and far lower than the risks for COVID-19 infection, it is higher than for the Moderna and Pfizer vaccines, which is why the CDC recommends that most people should get a Moderna or Pfizer vaccine. The identification of any possible risks, like the risks associated with the Johnson & Johnson vaccine, is a sign that the nation’s safety monitoring system for COVID-19 vaccines is working.

People who are not yet vaccinated, along with those who received the Johnson & Johnson vaccine and are now eligible for a booster shot, should get a Pfizer or Moderna vaccine. These two vaccines are widely available, but Johnson & Johnson vaccines are still on the market for cases where other vaccines are not accessible or clinically appropriate.

Updated May 6, 2022 

Data show an overall rate of 3.23 cases of the blood clotting disorder per 1 million Johnson & Johnson doses administered, and the onset of symptoms for the individuals typically occurred one to two weeks after getting the vaccine. Risk varies by age and gender, but data show that the risk of developing a rare blood clotting disorder after receiving the Johnson & Johnson vaccine is highest for women age 30-49. Most people should get a Pfizer or Moderna COVID-19 vaccine, with the use of Johnson & Johnson limited to adults age 18 and older in instances where other vaccines are not accessible or clinically inappropriate.

Updated May 6, 2022 

A booster dose is given after a complete vaccine series to provide additional protection against a vaccine’s effectiveness has decreased over time, while an additional dose is given to people with compromised immune systems to improve their response to the initial vaccine series. 

People with compromised immune systems may have a reduced ability to respond to vaccines, and having a weakened immune system can increase the risk of becoming severely ill from COVID-19. The CDC recommends that immunocompromised people who received the Pfizer or Moderna vaccine get an additional dose at least 28 days after their second shot. Data show that an additional dose of the Pfizer or Moderna vaccines helps to increase protection for this group. 

Patients who are immunocompromised should consult with their health care provider to discuss additional precautions and any questions they have about protecting themselves from COVID-19.

Updated February 28, 2022 

Emergency use authorization (EUA) allows the FDA to authorize the use of yet to be approved drugs, or unapproved uses of approved drugs, for life-threatening conditions when there are no other adequate, approved, and available options and other conditions are met. In the case of COVID-19, the FDA issued EUAs for the Pfizer, Moderna, and Johnson & Johnson vaccines, and has now issued full approval for the Pfizer and Moderna COVID-19 vaccines.

In an emergency when lives are at risk, like a pandemic, it may not be possible to have all the evidence that the FDA would usually have before approving a vaccine or drug. If there’s evidence that strongly suggests that patients have benefited from a treatment, the agency can issue an EUA to make it available. For the COVID-19 vaccines, FDA required two months of safety and efficacy data before the EUA was granted. That included clinical trials with tens of thousands of people and rigorous testing and review, and all the vaccines continue to be closely monitored. Compared to emergency use authorization, FDA approval of vaccines requires even more data on safety, manufacturing, and effectiveness over longer periods of time and includes real-world data.

Updated February 2, 2022 

The CDC recommends that most people get a Pfizer or Moderna COVID-19 vaccine over the Johnson & Johnson vaccine, but there is no preference between the Pfizer or Moderna vaccine. These two vaccines are widely available, and for most people, getting the first available COVID vaccine is the best thing you can do to safeguard your health. If you have additional questions about which vaccine is best for you, check with your doctor.

The Pfizer and Moderna COVID-19 vaccines have both received full FDA approval. Before receiving EUA, all three vaccines underwent rigorous testing and review—including clinical trials with tens of thousands of people, and the FDA evaluated comprehensive data on their safety and effectiveness. All three showed excellent safety and effectiveness profiles, and now, hundreds of millions of vaccine doses in the U.S. and billions worldwide have been safely administered. The Johnson & Johnson vaccines will continue to be safely administered through emergency use authorization as the FDA reviews data about their real-world use. 

February 2, 2021 

Vaccines are a safe and effective way to stop the spread of preventable diseases and decrease rates of infection, hospitalization, and death. The CDC recommends all people age 5 and older get vaccinated against COVID-19, and get a booster shot when eligible. Schools or employers may require vaccinations for attendance or employment, and requirements vary by state and employer. 

While vaccine requirements vary by state, location, business, and school, the science remains the same: the best way to protect yourself from getting COVID-19 is to get vaccinated, and boosted when eligible.

Schools: All states have vaccination requirements for children attending school and childcare facilities. Vaccination requirements help safeguard children by making sure they are protected when they begin school, where there is a higher potential for transmission of some diseases. To learn more about vaccine requirements by state, visit the CDC’s SchoolVaxView Requirements Database

Employers: On January 13, 2022, the Supreme Court ruled that the federal government cannot enforce a vaccine mandate for large businesses. This does not mean that private employers are blocked from creating vaccine mandates. Vaccines remain the safest and most effective way to protect against COVID-19, and employers are still legally able to mandate COVID-19 vaccinations for employees.

Other Vaccine Requirements:

  • Health care workers at facilities that receive Medicare and Medicaid funding are required to get a COVID-19 vaccine.
  • Businesses may also require patrons to show proof of vaccination for entry, and these requirements vary on the state and local level.


Rare adverse events associated with the Johnson & Johnson vaccine typically present within days or weeks following the shot. The risk of having an adverse reaction to the vaccine is very low, and even lower as time passes.

Data show that symptoms of a blood clotting disorder (severe headache or abdominal pain, shortness of breath, neurological symptoms, leg swelling) typically developed about nine days after vaccination, and in no cases did the onset of symptoms occur after 18 days. If you have questions or concerns, consult your doctor.

Updated December 17, 2021 

There are a few reasons why people who are vaccinated continue to get COVID-19. For one, no vaccine is 100% effective at preventing infection, and highly contagious variants have led to breakthrough infections among vaccinated people. Also, the level of protection from the vaccine decreases over time, leading to less protection against the virus. Ultimately, as the total number of vaccinated people increases, the pool of unvaccinated people gets smaller—that means proportionally, more cases will be among the vaccinated.

While vaccines are developed in part to prevent infection from disease, the main goal of vaccines is to prevent severe illness or death. The COVID-19 vaccines continue to be highly effective in reducing risk of severe disease, hospitalization, and death, and can provide sustained protection when you receive a booster dose. When COVID-19 cases rise, breakthrough infections among vaccinated individuals are significantly more likely to be mild cases, while unvaccinated people are more likely to become severely ill or require hospitalization. CDC data show that through December 25, 2021, the risk of being hospitalized with COVID-19 in the U.S. was 16 times greater for unvaccinated adults than fully vaccinated adults.

Updated February 18, 2022 

The COVID-19 vaccines have received the most intense safety monitoring in U.S. history, which has allowed public health officials to make science-based recommendations that keep people safe.

All COVID-19 vaccines have been rigorously tested and reviewed. The vaccine’s clinical trials three-phase process was detailed and thorough, and no shortcuts were taken. More than 150,000 people participated in U.S. clinical trials of the vaccines, and now, hundreds of millions of vaccine doses in the U.S. have been safely administered. Data from trial will continue to be collected for two years after each vaccine is first administered to ensure that they are safe for the long term. As with all vaccines, there will be ongoing monitoring for adverse events among people who are vaccinated into the future.

Updated October 12, 2021 

It may seem like the vaccines were developed quickly, but the process included rigorous safety reviews required for all new vaccines. The urgency of the pandemic created greater access to research funding, reduced bureaucratic obstacles, and encouraged unparalleled levels of government and industry cooperation. With these supports in place, scientists built upon previous work on coronavirus vaccines and on mRNA vaccine technology to develop these new vaccines quickly and effectively.

Updated October 12, 2021 

In general, you are considered fully vaccinated for COVID-19 two weeks after you have received the second dose in a two-dose series (Pfizer-BioNTech or Moderna) or two weeks after you have received a single-dose vaccine (Johnson & Johnson).

As the science and the virus evolve, so does our understanding of what it means to be fully vaccinated.  Scientists and medical experts continue to closely watch for signs of waning vaccine immunity over time, how well the vaccines protect against new variants of the virus, and how that data differs across the population.

While additional or booster doses are recommended for some people, the CDC definition of what it means to be “fully vaccinated” has not changed at this time. More messaging guidance about booster doses can be found here.

Updated October 12, 2021 

Yes. Based on data on the safety of COVID-19 vaccines during pregnancy, CDC recommends COVID-19 vaccination for all people who are pregnant, breastfeeding, or trying to get pregnant now or in the future. Data show that pregnant and recently pregnant people are more likely than non-pregnant people to get severely ill if they are infected with COVID-19, and the highly contagious Delta and Omicron variants makes it even more important for eligible people to get vaccinated. 

In addition, the American College of Obstetricians and Gynecologists and other leading maternal health and public health organizations are “strongly urging” all pregnant individuals, and anyone planning to become pregnant, to get vaccinated against COVID-19. 

Updated February 18, 2022 

No. There is no evidence to show that getting a vaccine increases the risk of miscarriage.

There has been extensive safety monitoring of the COVID-19 vaccines, including analysis of vaccination during pregnancy. Specifically, studies show that the rate of miscarriage in the first 20 weeks of pregnancy in the general population is about 11-16%, and an analysis of safety monitoring data of people who received an mRNA COVID-19 vaccine showed a similar rate of 13%.In other words, being vaccinated with one of the currently available COVID-19 vaccines does not increase miscarriage risk; rather, it protects against the higher risk of serious illness if you are pregnant and become infected with the virus.

Added August 12, 2021 

No. There is no evidence that fertility problems are a side effect of any vaccine, including COVID-19 vaccines.

Added August 12, 2021 

Antibodies made after a pregnant person received an mRNA COVID-19 vaccine have been found in umbilical cord blood, which means that COVID-19 vaccination during pregnancy might help protect babies against COVID-19. Additionally, recent reports have shown that breastfeeding people who have received mRNA COVID-19 vaccines have antibodies in their breast milk, which could help protect their babies. In both of these cases, more data are needed to determine the level of protection these antibodies may provide to the baby and how long that protection would last.

Added August 12, 2021 

The CDC recommends all people age 5 and older get vaccinated against COVID-19, including people who were previously infected with the virus. Data show that immunity in people who have been infected with COVID-19 wanes over time, and scientists continue to study this. New data show that COVID-19 vaccination can provide a higher, more robust, and more consistent level of immunity to protect people from COVID-19 than antibodies from infection alone. 

COVID-19 vaccination is effective in preventing reinfection in people who previously had COVID-19. One study, for example, showed that among people hospitalized with COVID-19, those who were previously infected with COVID-19 were 5 times more likely to get COVID-19 again if they were unvaccinated than people who were fully vaccinated. For that reason, even if you have already had COVID-19, vaccination is an important step to protect yourself and those around you.

Updated November 15, 2021 

While COVID-19 vaccines are highly effective, no vaccine provides 100% immunity. Because this is a new virus, scientists and medical experts continue to monitor how long immunity lasts, whether some groups may need additional doses, and how well the vaccines protect against new variants of the virus.

Data continue to show that the COVID-19 vaccines are extremely effective in protecting fully vaccinated people from catching and spreading the virus, including the Delta variant, and scientists continue to monitor vaccine efficacy for new variants. A small percentage of vaccinated people experience breakthrough cases, but they are much more likely to have milder symptoms than unvaccinated people who get COVID-19.

Unvaccinated people continue to account for the vast majority of severe cases, hospitalizations, and deaths from COVID-19. CDC data show that in August 2021, the risk of dying from COVID-19 in the U.S. was more than 11 times greater for unvaccinated people than for fully vaccinated people.

The risk of severe illness from COVID-19 is elevated for some groups — including older adults, people with underlying medical conditions, immunocompromised people, and pregnant or recently pregnant women. If you have questions about your risk of COVID-19, how to protect yourself, or the vaccines, speak to your health care provider.

Updated December 9, 2021 

The threat of COVID-19 is real and urgent, and getting vaccinated is the best way to protect yourself. Side effects to the COVID-19 vaccines are typically mild and subside in one to two days — like soreness in the arm, fatigue, headaches, or a slight fever.

The risk of having a serious adverse reaction to the COVID-19 vaccine is very low — far lower than the risk of contracting COVID-19. The CDC and FDA are closely monitoring vaccine outcomes to ensure safety.

If you have a question about the vaccines, talk with your healthcare provider.

Updated October 12, 2021 

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