If you have a moderately to severely weakened immune system and received either the Pfizer or Moderna vaccines for your first two doses, you should get an additional (third) primary dose of the same vaccine 28 days after the second dose. People age 5-17 who are moderately to severely immunocompromised can only get the Pfizer vaccine as an additional (third) primary dose. The CDC does not recommend mixing products for a two-dose primary series or additional primary doses.
If you are immunocompromised and you received the Johnson & Johnson vaccine, you should receive an additional (second) dose of a Pfizer or Moderna vaccine 4 weeks later.
If you became immunocompromised AFTER receiving valid primary series doses of the Pfizer or Moderna vaccine (or 1 Johnson & Johnson vaccine) and a valid booster dose, it is not recommended that you receive an additional dose of vaccine at this time.
If you are eligible for an additional primary dose, you should get it before you get a booster shot. If you are immunocompromised and got 3 primary doses of Pfizer, you should get a booster (fourth) dose 3 months later. If you are immunocompromised and got 3 doses of Moderna, you should get a booster (fourth) dose 3 months later. If you are immunocompromised and got a single initial dose of Johnson & Johnson followed by an mRNA (Pfizer or Moderna) dose, you should get a booster (third) dose 2 months later.
We encourage you to speak with your health care provider if you have questions about whether you are eligible to receive an additional primary dose.