COVID-19 Vaccine Booster: What You Need to Know

Medically Reviewed By William C. Lloyd III, MD, FACS
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woman with face mask getting vaccinated for covid-19 at a clinic with other people waiting their turn

Do you need a COVID-19 vaccine booster shot?

Most likely. It’s been less than a year since the first COVID-19 vaccines were authorized for use in the United States, so scientists are still learning about immunity after vaccination. They know vaccination remains protective against severe illness, but immunity from infection and symptoms decreases over time. The rise in breakthrough COVID-19 (infections or symptomatic disease in fully vaccinated people) was and is still likely due to the more infectious and contagious Delta variant, and now the Omicron variant is on the scene.

With clear signs of waning immunity, the Centers for Disease Control and Prevention (CDC) is continuously working on how to keep people safe from variants of concern like Delta and Omicron. Booster shots seem to be the answer. Booster shots—additional doses of vaccine after initial immunization—can “boost” immunity to disease by further stimulating the immune system. Adults get tetanus booster shots every 10 years to maintain sufficient levels of immunity to tetanus; if you skip your booster shot, you may be more likely to contract tetanus (also called lockjaw) if you step on a rusty nail, for instance.

Here's what you need to know about COVID-19 vaccine boosters and eligibility.

Anyone 12 and older can get a COVID-19 vaccine booster shot.  

Results from Pfizer-BioNTech COVID-19 vaccine studies show that a third dose of the vaccine significantly increased antibody levels in people who were fully vaccinated six months prior. A third dose of vaccine also improves protection against the Delta variant. Preliminary studies show that the immune response from a third dose of the Pfizer vaccine can block the Omicron variant too. Getting a booster shot offers good protection against serious COVID-19 requiring hospitalization.

Booster shot recipients report fewer side effects compared to the first and second shots and no serious adverse events. (Pfizer and BioNTech are also developing a booster vaccine that’s made to specifically target the entire spike protein of the Delta variant.)

Pfizer-BioNTech booster shot eligibility

In August 2021, the FDA authorized a booster dose of the Pfizer vaccine for people ages 18 and older at least six months after initial vaccination with the Pfizer vaccine. Since August, the FDA authorized the Pfizer booster for 12- and 17-year-olds. The FDA also updated the emergency use authorization of COVID-19 booster, shortening the time between primary vaccination (the first two shots) and the third shot (booster) to five months. (Children ages 5 to 11 years who are immunocompromised may receive an additional dose of the Pfizer-BioNTech COVID-19 vaccine; see below.)

To help stop the spread of SARS-CoV-2, the CDC recommends that everyone get a booster shot if it has been at least six months since their primary vaccination.

The CDC notes that people particularly at risk include:

  • People aged 65 years and older
  • People aged 18 to 64 years with underlying medical conditions, including pregnancy
  • People aged 18 to 64 years living or working in an environment that increases exposure to SARS-CoV-2, the virus that causes COVID-19. This includes long-term care settings, healthcare settings, and institutional settings like prisons.

Moderna booster shot eligibility

People ages 18 and older can receive a booster dose of the Moderna vaccine if it has been at least six months since their primary vaccination series. The Moderna booster shot is half the dose of the first two shots.

    Johnson and Johnson (J&J) booster shot eligibility

    The FDA also authorized a Johnson & Johnson (J&J) booster dose for people 18 and older at least two months after initial vaccination with the J&J vaccine. The authorization does not restrict the J&J booster dose to at-risk populations, so anyone who received the single-shot J&J vaccine may receive a second (booster) dose two or more months later.

    You can mix and match COVID-19 vaccine booster doses.

    The FDA also authorized a "mix and match" approach to boosters. This means you may receive a booster dose of a vaccine that is different from the one you received initially. The eligible population(s) and dosing schedule vary depending on the vaccine used for primary vaccination. For example, recipients of the single-shot J&J vaccine may receive the Pfizer or Moderna booster at least two months after primary vaccination with the J&J vaccine. In contrast, recipients of the two-shot Moderna or Pfizer COVID-19 vaccine who are eligible for a booster must wait six months after primary vaccination.

    Booster shots are available at most major pharmacies. Search, text your ZIP code to 438829, or call 1-800-232-0233 to find locations near you. Bring your vaccination card with you. You should have received a vaccination card when you got your first two vaccine shots (or one shot of the J&J vaccine). It tells you what type of vaccine you received and when and where you were vaccinated. If you do not have your vaccination card, contact the vaccination site where you were vaccinated or your state health department.

    Immunocompromised people can get a booster dose two months after initial vaccination.

    Research has shown that immune-compromised people aren’t as well protected after COVID-19 vaccination as people with healthy immune systems. One small study found that 40 to 44% of people hospitalized with breakthrough COVID-19 were immunocompromised individuals. By reintroducing the target antigen (with an additional dose of vaccine), individuals who have minimal immunity can greatly elevate the speed and intensity of their own host immune response.

    On August 12, 2021, the FDA authorized an additional COVID-19 vaccine dose—a third shot of either the Pfizer-BioNTech or Moderna mRNA vaccine—for people who received organ transplants or have a weakened immune system due to disease or medical treatment.

    Immunocompromised adults 18 years of age and older may receive either of these mRNA vaccines. People 5 years of age or older may receive an additional dose of the Pfizer-BioNTech vaccine only.

    The CDC recommends an additional dose of vaccine for people who:

    • Have been receiving cancer treatment
    • Received an organ transplant and are taking anti-rejection medication
    • Received a stem cell transplant within the last two years
    • Have moderate or severe primary immunodeficiency
    • Have advanced or untreated HIV infection
    • Are taking medication that suppresses the immune system

    Unlike a booster shot, the additional dose can be administered four weeks (28 days) after the second dose of vaccine. Talk with your doctor if you are unsure of your immune health.

    Ideally, the third shot should be the same vaccine as the first two doses. So, an individual who previously received two doses of the Moderna vaccine should get an additional Moderna dose, though another brand of mRNA COVID-19 vaccine may be administered if the original vaccine is not available.

    A doctor’s note verifying an immunocompromised state is not required to receive a third dose of vaccine, but you must declare you meet at least one of the criteria listed.

    Note: The FDA has not yet authorized an additional dose of the J&J vaccine specifically for immunocompromised people; however, any recipient of the J&J COVID-19 vaccine, regardless of immune health, is eligible for a booster dose of the J&J vaccine two months after the initial J&J shot or a booster dose of the Pfizer or Moderna vaccine six months after the initial J&J shot. This guideline may change as more information about the J&J vaccine and immunocompromised people is available.

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    Medical Reviewer: William C. Lloyd III, MD, FACS
    Last Review Date: 2021 Aug 18
    View All Vaccines Articles
    THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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