COVID Vaccine Booster: Everything You Need to Know

Medically Reviewed by Brunilda Nazario, MD on September 20, 2021

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. Getting another jab of the vaccine 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. Both Pfizer and Moderna are mRNA COVID-19 vaccines.

The CDC recommends booster shots for fully vaccinated people. The timing depends on which vaccine you receive at first.

Pfizer-BioNTech. After you’ve got the two jabs required for the primary series of the mRNA vaccine, people 12 or older are eligible for a booster shot 5 months after the last dose.

People 12-17 years old may only get a Pfizer booster. Everyone else can get booster shots from either Pfizer or Moderna.

Moderna. After the first series, anyone 18 or older can get a booster shot 5 months after the last dose.

Johnson & Johnson’s Janssen. Those 18 and over are eligible for a booster shot, preferably from either Pfizer or Moderna, 2 months after the first dose.

Additional Boosters

The CDC recommends an additional booster shot for certain people to amp up their immune system against potential COVID-19 variants.

If you have a weakened immune system or are 50 or above, you’re eligible for a second mRNA booster shot 4 months after your first booster.

If you got the J&J Janssen vaccine as your first vaccine and first booster dose, the CDC recommends a second booster using an mRNA COVID-19 vaccine. You may get this at least 4 months after your last dose.

What Else Does Research Say About COVID-19 Boosters?

Boosters are 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, you’re eligible for an mRNA booster shot 3 months later. And the CDC recommends a second booster shot 4 months after you’ve got the first booster.

Show Sources


CDC: “CDC Expands Eligibility for COVID-19 Booster Shots,” “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,” “CDC Recommends Additional Boosters for Certain Individuals,” “COVID-19 Vaccines for Moderately or Severely Immunocompromised People,” “COVID-19 Vaccine Booster Shots.”

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: “Coronavirus (COVID-19) Update: FDA Takes Additional Actions on the Use of a Booster Dose for COVID-19 Vaccines,” “Vaccines and Related Biological Products Advisory Committee Meeting, September 17, 2021.”

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