Viruses commonly change a little bit over time. This is common and expected. How much they change and how frequently they change depends a lot on how quickly the virus is spreading from person-to-person. The more quickly a virus spreads, the more the virus has a chance to change. When the virus changes, it may be called a new variant.
Variants are expected. The best way to slow the emergence of new variants is to reduce the spread of infection by taking measures to protect yourself, including getting a COVID-19 vaccine when available.
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 cause most new infections. 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 newer Omicron variants.
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.