A booster dose is another dose of a vaccine that is given to someone who built enough protection after vaccination, but then that protection decreased over time (this is called waning immunity). The CDC recommends people 12 and older receive an age-appropriate, updated mRNA booster dose if at least 2 months have passed since their primary series or since their most recent booster dose. Updated boosters of Pfizer-BioNTech and Moderna contain a bivalent formula that both boosts immunity against the original coronavirus strain and also protects against the newer Omicron variants that now cause most cases. A person can receive a booster of a COVID-19 vaccine that is different than the COVID-19 vaccine they received for their primary series or initial booster.

CDC recommends children ages 5 to 11 who received the Pfizer-BioNTech vaccine receive the same booster as their primary vaccine. Children under 5 who received the Pfizer vaccine and children under 12 who received the Moderna vaccine are not yet eligible for a booster. The Johnson & Johnson vaccine is authorized for anyone age 18 and older if no other approved vaccine is available.