A “third dose” of an mRNA vaccine 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.
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.
Visit the Centers for Disease Control and Prevention site for the latest additional dose information for immunocompromised individuals.
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.
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.
Reactions reported after an additional 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.
