Another study, conducted by the C.D.C., suggested that the updated booster shot performed as well against the newest Omicron subvariant, XBB.1.5, as it did against the earlier version of Omicron, BA.5. For adults under 50, a bivalent booster was 48 percent effective against Covid infections caused by XBB.1.5, the C.D.C. found, and 52 percent effective against infections caused by BA.5.
Those results could have been skewed, however, by a number of factors. For example, the results from the C.D.C. study could have been affected by rates of previous infection among participants.
Officials said that factor could not be accurately measured, but how well the bivalent vaccine worked would have been understated, not overstated, as a result. Still, they acknowledged that data on how long the added protection lasted was limited to a few months.
Overall, health officials cast the data as an encouraging sign that the updated boosters were holding up against the latest variants, even though they were designed to counter versions of the virus that started disappearing this winter.
As of November, C.D.C. officials said on Thursday, people with a bivalent booster were experiencing a 13-fold reduction in the risk of death from Covid compared with unvaccinated people, and a twofold reduction in the risk of death compared with people who were vaccinated but had not received an updated booster.
Pressed by the expert committee on Thursday, F.D.A. scientists also weighed in on the contentious question of whether future updated vaccines should be designed in part against the original version of the virus, as the current bivalent boosters are, or against a new version of the virus. They indicated that future vaccine formulations could be aimed exclusively at new variants, rather than the original version of the virus.
F.D.A. officials told the committee that an extensive investigation had alleviated worries that the updated booster shot made by Pfizer-BioNTech could increase the risk of strokes in people 65 and older. One surveillance system suggested a possible heightened risk. But federal regulators said other major safety monitoring systems failed to validate that concern and there was no reason not to recommend the shot.
Christina Jewett contributed reporting.
