FAQ: Booster vs Third Dose Eligibility

Booster or third dose?


  • Helps boost waning immunity
  • Given at least 5 months after initial Pfizer & Moderna doses or 2 months after the J&J shot
  • Approved for all adults, 5+ for Pfizer
  • Pfizer, Moderna and Johnson & Johnson

Third dose

  • Helps certain people reach a stronger level of immunity not reached after 2 doses
  • Given 28 days after the second vaccine dose
  • For those with certain immunocompromising conditions
  • Pfizer and Moderna


As the eligibility for the COVID-19 vaccine evolves, you may be wondering if you should get an additional dose. We answered some frequently asked questions below. 

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

A “third dose” of an mRNA vaccine (either Moderna or Pfizer) is available to people who may not have had a strong enough immune response after receiving the first two doses. For example, those who are immunocompromised need a third dose to reach a stronger level of immunity against the virus. 

A “booster” is an additional vaccine dose given to individuals who may have waning immunity because they completed their initial vaccine series many months ago. The rise of variants, such as the highly contagious Omicron variant, is another reason why some people may be mounting a less robust immune response. 

Who is eligible for a third dose?

A third dose of the Pfizer or Moderna vaccine is available for people with certain immunocompromised conditions including current treatment for cancer, those who received an organ transplant or those with advanced or untreated HIV.

Learn more about the eligible conditions for third doses.

Who is eligible for a booster?

Booster doses are approved for all adults. Those 5 and older can receive a Pfizer booster. If you are eligible, you can get your booster of the Pfizer or Moderna vaccine if it has been at least five months since your first two doses. If you received the Johnson & Johnson vaccine, you should receive a booster if it has been at least two months since your shot. While mRNA vaccines are recommended as a booster for those who received a Johnson & Johnson vaccine, a Johnson & Johnson booster can be considered if you have a severe allergy to any ingredients in the Pfizer or Moderna vaccines. 

Those who are immunocompromised should receive their booster dose if it has been at least three months since their vaccine series. Those 50 and older and those with certain immunocompromising conditions can receive a second booster of the Pfizer or Moderna vaccines four months after their initial booster dose.


Can I "mix and match" vaccines for my booster shot?

The CDC now allows for recipients to "mix and match" their booster shot. That means if your first two doses were with the Pfizer vaccine, you can receive a booster dose with the Moderna vaccine. If you received a Johnson & Johnson vaccine, you are eligible to receive a Pfizer or Moderna booster shot.

I do not have any underlying conditions. Why would I need a booster?

The available data shows people who got their initial doses of the vaccine may have waning immunity after a period of time. A booster shot can help boost protection against COVID-19.

I am eligible for a third dose but received the single dose J&J/Janssen vaccine. Should I get a second dose?

Those who are immunocompromised should not receive a second dose of the Johnson & Johnson vaccine. However, they should receive a booster shot if it has been at least two months since their primary vaccination.

Will I experience the same side effects for this extra dose as I did for my previous shots?

So far, reactions reported after the third dose were similar to that of the two-dose series: fatigue and pain at injection site were the most commonly reported side effects, and overall, most symptoms were mild to moderate. However, as with the two-dose series, serious side effects are rare, but may occur.