To be considered fully vaccinated, the Comirnaty (Pfizer-BioNTech), Spikevax (Moderna) and Novavax vaccine require two doses. The Johnson & Johnson (Janssen) vaccine requires one dose. The Pfizer-BioNTech vaccine for children 6 months to 4 years old requires 3 doses. See additional recommendations for people who are moderately to severely immunocompromised.
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 overtime (this is called waning immunity). The CDC recommends people 5 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 cause most cases.
Booster shots are not available for children under 5. The monovalent Novavax booster and the Johnson & Johnson primary and booster vaccinations are authorized for people 18 and older in limited situations.