The majority of the coronavirus strains infecting people right now are either descended from, or related to, XBB.1.5, so the decision to target that variant with the vaccine “was about as good as you could imagine for the moment,” said Trevor Bedford, a professor in the vaccine and infectious disease division at Fred Hutchinson Cancer Research Center.

The vaccine will most likely also provide some protection against EG.5, which recently became the dominant variant in the United States, accounting for about 17 percent of current cases. EG.5 is descended from another XBB variant and has a few additional mutations, so antibodies produced by the updated vaccine may not be quite as effective against it. But the new booster is still a better fit for EG.5 than last year’s booster, which targeted both the original Covid strain and the BA.5 Omicron variant — neither of which appear to be circulating anymore.

Dr. David Boulware, a professor of medicine specializing in infectious diseases at the University of Minnesota Medical School, added that because the new vaccine is a better match for the current variants, he is “somewhat optimistic” that it will help prevent not only severe disease but also infection.