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.An additional dose is not authorized for those who received a Novavax primary series.

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. For booster vaccination, CDC recommends eligible people age 5 and older receive an age-appropriate, updated Pfizer or Moderna vaccine that offers better protection against variants. The monovalent Novavax booster and the Johnson & Johnson primary and booster vaccinations are authorized for people 18 and older in limited situations.

See additional recommendations for people who are moderately to severely immunocompromised.