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When the mRNA COVID-19 vaccines were first unveiled in December 2020, medical experts touted the benefits of this new technology, saying formulations could easily be tweaked someday to match a quickly changing virus. That day has finally come.

In early September, the Food and Drug Administration (FDA) authorized and the Centers for Disease Control and Prevention (CDC) recommended two updated boosters. The boosters target two Omicron subvariants, BA.4 and BA.5. These are by far the most contagious versions of SARS-CoV-2 to date, with BA.5 accounting for nearly 90% of cases in the US

While the original mRNA coronavirus vaccines have proven effective at preventing death and severe disease from COVID-19, breakthrough infections and re-infections have become more common in the face of an evolving virus.

The boosters (one from Pfizer-BioNTech for ages 12 and up; the other from Moderna for ages 18 and up) are authorized under an emergency use authorization (EUA).

We asked Yale Medicine infectious diseases experts to answer common questions about these new reformulated boosters.