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 two formulas that boost immunity against the original coronavirus strain and also protect against the newer Omicron variants. Updated boosters are expected to provide better protection against the virus and address waning vaccine effectiveness over time.
Note that 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 recommended vaccination schedule from the Centers for Disease Control and Prevention, including dosage, is available here.