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). To date, booster doses have worked well in extending the protection of the vaccine against serious illness but have been somewhat less effective in boosting immunity against new variants of COVID-19 compared to the original strain. Pfizer-BioNTech and Moderna have each created an updated booster dose formula designed to protect against original strains of the virus, as well as Omicron variants that now cause most cases.
The CDC recommends people 6 months 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 Omicron variants that now cause most cases. For children age 6 months to 4 years who get the Pfizer primary series, the updated bivalent vaccine will be used as the third dose in the series, rather than as a separate booster. Children in this age group who previously received 3 monovalent Pfizer primary series doses are eligible to receive 1 bivalent Pfizer booster dose at least 2 months after completion of the monovalent primary series.
The latest CDC recommendations on booster doses help to ensure more people across the U.S. are better protected against COVID-19. The best way to protect yourself from COVID-19 is to get vaccinated and boosted if eligible. Vaccination and boosting are particularly important for individuals more at risk for severe COVID-19, such as older people and those with underlying medical conditions.