All Adults Are Eligible for COVID-19 Booster: What to Know

MSK healthcare worker administering a COVID-19 shot to a patient

Everyone over 18 is eligible for a booster dose of any of the three COVID-19 vaccines available in the US.

The US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) are now recommending that everyone over age 18 who received a COVID-19 vaccine receive a booster dose in order to provide greater protection against severe illness from the disease.

The CDC endorses booster shots of the Pfizer-BioNTech, Moderna, and Johnson & Johnson/Janssen (J&J) COVID-19 vaccines. The CDC also endorses the mix-and-match approach to boosters, saying eligible people can choose whichever vaccine they wish as a booster.

Mini Kamboj, Memorial Sloan Kettering Cancer Center’s Chief Medical Epidemiologist, has answers to your questions about who is eligible and how you can schedule an appointment.

Who is eligible to receive a COVID-19 booster dose?

For patients who received the Pfizer-BioNTech or Moderna vaccine, the FDA and CDC recommend that everyone over age 18 get a booster shot.


People who received the Moderna vaccine are eligible for either a half-dose of the Moderna vaccine or a full dose of Pfizer-BioNTech or J&J. They can receive a booster dose six months after their second shot of Moderna.


People who received the Pfizer vaccine are eligible for a booster dose of Pfizer or J&J, or they may receive a half-dose of the Moderna vaccine. They may receive a booster dose six months after their second shot of Pfizer.  

Johnson & Johnson/Janssen

People who received the J&J vaccine are eligible for a Pfizer or J&J booster, or half-dose of Moderna booster. These people are eligible for a booster shot two months after receiving the J&J vaccine.  

If you received both doses of the Moderna or Pfizer vaccine plus a booster shot, or a single dose of the J&J vaccine plus a booster shot, the FDA and CDC do not recommend that you receive any additional COVID-19 vaccine shots.

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Why is the Moderna booster shot a half-dose? 

According to data submitted to the FDA and CDC by Moderna, a half-dose booster shot of its COVID-19 vaccine effectively strengthens a person’s immunity against COVID-19 and its variants. In addition, Moderna stated that giving patients a 50-microgram booster shot — half the dose of the original two shots — may reduce side effects a person can experience, such as fatigue, headache, body aches, and fever. The FDA and CDC did not approve a 100-microgram booster dose of the Moderna vaccine.

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Is it safe and effective to mix-and-match vaccine brands?

Yes, a different type of vaccine to boost immune response after the initial vaccination is safe and effective. Federal health officials endorsed this approach, especially for situations where the original vaccine may not be available. It is important to remember that the data on this approach is still limited, and health officials did not recommend one vaccine brand over another as a booster.

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When will MSK be offering these additional booster shots?

MSK is offering additional booster shots now. To check your eligibility and to schedule a booster vaccination at MSK, please use this link. If you have questions about the booster shot, please call your MSK doctor’s office.

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Can I get my booster dose outside of MSK?

Yes. Patients don’t need wait to get a booster shot at MSK. We encourage them to look for a vaccination location with availability near you, using the following links:

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I am getting a COVID-19 vaccine booster dose at MSK. What do I need to bring to my appointment?

Please bring your CDC-issued COVID-19 vaccination card(s) to your booster dose appointment. This is important for us to confirm the date of your last dose. We will update your vaccination card with your booster dose information.

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What side effects should I anticipate if I get a COVID-19 booster dose?

The side effects from an additional dose of the COVID-19 vaccine are similar to those experienced after receiving the original vaccines; the most common are soreness at the injection site, fatigue, headache, body aches, and fever. These symptoms don’t last long typically — about one to three days.

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Is it safe for pregnant people to get a booster shot?

Yes, a growing amount of data continues to confirm that COVID-19 vaccines are safe during pregnancy. On Sept. 29, 2021, the CDC issued an “urgent health advisory,” calling on pregnant people to get vaccinated against COVID-19. The American College of Obstetricians and Gynecologists (AGOC) followed suit on Oct. 1, and encouraged pregnant people who received the Pfizer vaccine at least six months ago to get a booster dose.

The organizations made their recommendations after hospitals nationwide saw a sudden uptick in unvaccinated pregnant women becoming severely ill or hospitalized with COVID-19.  People who are pregnant, unvaccinated against COVID-19, and contract the virus are at higher risk of serious complications, including being admitted to the ICU, needing a breathing tube, and even death. They may also be at higher risk of adverse pregnancy outcomes, such as preterm birth and stillbirth.

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If public health experts are recommending a booster dose, does this mean the COVID-19 vaccines aren’t working?

No. Immune protection tends to weaken over time, and a booster dose can help train the immune system to continue recognizing the virus to protect the body against it. For example, an additional dose of the tetanus and diphtheria vaccine is recommended every 10 years.

The COVID-19 vaccines continue to be very effective in preventing severe illness, hospitalization, and death, including against the highly contagious Delta variant. However, studies show fully vaccinated people have begun to experience reduced protection against mild and moderate infection from COVID-19 and its variants, including Delta. For that reason, people who are eligible should get a COVID-19 booster dose to enhance their protection.

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Is it safe to receive a flu shot and a COVID-19 vaccine booster dose at the same time?

Yes, it is safe to receive a booster dose of the COVID-19 vaccine and other vaccines — such as a flu shot — during the same visit. MSK’s medical experts encourage patients to get both — it is important to protect yourself from both COVID-19 and the flu. If you do get a flu shot and a COVID-19 booster dose at the same time, the CDC advises receiving the injections in different arms to avoid soreness.

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What is the difference between a booster dose and a third dose?

A booster dose is given to someone whose immune system has built excellent immunity after receiving the initial vaccine, but then that protection decreased naturally over time (this is called waning immunity).

A third dose of the vaccine is given to people who may not have achieved strong protection when they were first vaccinated. A vaccine works by activating the immune system. In some people with moderately to severely weakened immune systems (immunocompromised), this ability is impaired, so a third dose of a vaccine can help their immune systems build more protection against a disease.

A small group of these patients became eligible for a third dose of the Moderna and Pfizer vaccines based on recommendations from the FDA and the CDC’s independent advisory board.

These patients may receive a third dose as early as 28 days after the second COVID-19 vaccine dose, instead of waiting 6 months for a booster. The third dose is also a full dose of the vaccine for both Pfizer and Moderna.

For a complete list of eligible patients, click here.

November 23, 2021

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