Sometimes people who are moderately to severely immunocompromised do not build enough (or any) protection when they first get a vaccination. When this happens, getting another dose of the vaccine can sometimes help them build more protection against the disease, including COVID-19. Moderately or severely immunocompromised people 6 months or older who received an initial Moderna COVID-19 vaccine primary series should receive an additional primary dose of Moderna at least 28 days after the second dose. Moderately or severely immunocompromised people age 5 or older who received an initial Pfizer COVID-19 vaccine primary series should receive an additional primary dose of Pfizer at least 28 days after the second dose. Moderately or severely immunocompromised people age 18 or older who received the Johnson & Johnson vaccine should receive a single dose of an mRNA vaccine (Pfizer or Moderna) at least 28 days after their first dose.

In contrast, a booster dose refers to another dose of a vaccine that is given to someone who built enough protection after vaccination, but then that protection decreased over time and needs a boost. The Centers for Disease Control and Prevention has recommended booster doses for everyone 5 and older. For primary and booster vaccination, people should receive an age-appropriate mRNA vaccine (Pfizer or Moderna). The Johnson & Johnson vaccine is authorized for anyone age 18 and older if no other approved vaccine is available.

See additional recommendations for people who are moderately to severely immunocompromised. A second booster dose is recommended for certain people at increased risk of severe disease.