COVID Vaccine Booster: Everything You Need to Know

Do You Need a COVID-19 Booster Shot?

Most people who are fully vaccinated against the coronavirus are well-protected from serious illness and death from COVID-19.

But even highly effective vaccines often become less so over time. Early research on vaccines that use mRNA to protect you from the coronavirus, like those from Pfizer and Moderna, suggest that they eventually lose some of their power against infection and serious illness, no matter the variant of the virus (like Alpha, Beta, or Delta). Getting another shot several months after the first round, called a “booster shot,” can help supercharge the vaccine’s effectiveness.

The U.S. government says it plans to offer booster shots for people 18 or older who got two doses of the Pfizer or Moderna vaccines. The surgeon general recommends that these shots be timed for about 8 months after the second dose of vaccine.

But some people got a single-dose vaccine from Johnson & Johnson, which does not use mRNA. Scientists are still studying the timing and nature of a booster shot for these people, in part because the jab became available only in March 2021.

What Does Research Say About COVID-19 Boosters?

Early studies suggest a number of reasons to get a COVID-19 vaccine booster.

Most important, these vaccines are currently the best defense, by far, for you and the people around you. It’s important to get vaccinated right away if you qualify, as most people 12 and older do.

Research has found that:

  • Higher antibody levels from these vaccines make the average person less likely to get sick from the coronavirus (vaccine efficacy).
  • Antibodies for the Pfizer and Moderna vaccines decline over time for all known variants (Alpha, Delta, etc.). This is quite common for vaccines in general.
  • Booster shots of mRNA coronavirus vaccines (Pfizer and Moderna) increase antibodies by about 10 times and will almost certainly offer more protection against illness.
  • Higher levels of antibodies seem to be especially important against the Delta variant of the coronavirus. Infections from this variant are surging, and it seems to be more contagious, especially among children.


What if You Have a Weakened Immune System?

The CDC already suggests a third dose of mRNA vaccine for people with weaker immune systems, especially those who are “moderately to severely immunocompromised.” This includes those who are getting cancer treatment, who have had a stem cell or organ transplant, who have advanced or untreated HIV, or who are taking certain medications.

This third dose is not the same as a booster shot, which is specifically meant to “boost” the slowly waning effects of a vaccine.

Rather, this extra dose, typically given a month or so after the second dose, is meant to increase the first immune response because:

  • People with weaker immune systems are more likely to have serious, long-term illness from COVID-19.
  • A weaker immune system may not respond as strongly to the vaccine and so may not make enough antibodies to fight off infection and serious illness from COVID-19.
  • Even with a good vaccine response, people with weaker immune systems may benefit from extra protection against COVID-19.

If you have a weakened immune system and have had a third dose of the Pfizer or Moderna vaccine, talk to your doctor about whether you might need a booster shot.

WebMD Medical Reference Reviewed by Neha Pathak, MD on August 18, 2021



CDC: “Media Statement from CDC Director Rochelle P. Walensky, MD, MPH, on Signing the Advisory Committee on Immunization Practices’ Recommendation for an Additional Dose of an mRNA COVID-19 Vaccine in Moderately to Severely Immunocompromised People,” “COVID-19 Vaccines for Moderately to Severely Immunocompromised People,” “Talking with Patients Who Are Immunocompromised.”   

U.S. Office of the Surgeon General.

Anthony Fauci, MD, director, National Institute of Allergy and Infectious Diseases.

© 2021 WebMD, LLC. All rights reserved.