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 relatively 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.

But different vaccines may call for different booster timing. For example, research from the CDC shows that the Pfizer vaccine remains 91% effective at 4 months but drops soon afterward to 77%. The Moderna vaccine, on the other hand, appears to remain 92% effective even almost 5 months after vaccination.

The CDC recommends that anyone who got the Pfizer vaccine at least 6 months ago should get a booster shot if they are also:    

The CDC recommends that anyone who got the Pfizer vaccine at least 6 months ago may get a booster shot if they are also:

  • 18-49 years old with an underlying medical condition (based on risks vs. benefits, talk to your doctor)
  • 18-64 years of age and with “increased risk for COVID-19 exposure and transmission” because of work or “institutional setting” (based on risks vs. benefits, talk to your doctor)

What Does Research Say About COVID-19 Boosters?

Boosters are just a type of vaccine, and scientists already know that vaccines are currently the best defense against COVID, especially its most serious effects. That’s why it’s important to get vaccinated right away if you qualify, as most people 12 and older do.

Research also has found that:

  • In general, antibodies from vaccines decline over time.
  • Higher antibody levels from vaccines seem to make the average person less likely to get sick from the coronavirus (vaccine efficacy).
  • Booster shots of mRNA coronavirus vaccines (Pfizer and Moderna) increase antibodies by about 10 times.
  • 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.

Though not technically a booster shot, many people still call it that.

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 September 24, 2021



CDC: “COVID-19 Vaccine Booster Shot,” “CDC Statement on ACIP Booster Recommendations,” “Comparative Effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) Vaccines in Preventing COVID-19 Hospitalizations Among Adults Without Immunocompromising Conditions -- United States, March-August 2021,” “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. “FDA VRBPAC Votes to Recommend COVID-19 Boosters for Older Adults and High-Risk Groups.”

FDA: “Vaccines and Related Biological Products Advisory Committee Meeting, September 17, 2021.”

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