At the committee meeting on Thursday, the advisers wrestled with how frequently to offer shots, what variants to defend against, and whether the vaccines were meant to stop only hospital admissions or infections altogether.
Some experts said that it was too soon to say whether annual doses were needed, as they are for the flu.
“I think we need to see what happens with disease burden,” said Dr. Cody Meissner, the director of pediatric infectious disease at Tufts University School of Medicine. “We may or may not need annual vaccination. It’s just awfully early, it seems to me, in the process to answer that question.”
Several new studies in recent days have reported results that, while riddled with gaps in the data and other uncertainties, their authors said reinforced the benefits of the updated bivalent booster shots. Those vaccines were designed to defend against both the original version of the virus and the BA.5 Omicron subvariant that spread across the United States in the fall.
One of them, published in the New England Journal of Medicine, found that the bivalent boosters generated stronger protection than the original formulations of booster shots did.
The so-called bivalent booster was about 62 percent effective against severe Covid, the study found, compared with the original booster, which was 25 percent effective. (The study examined the original booster during last summer, and the updated booster during the fall.)
Another study, conducted by the Centers for Disease Control and Prevention, 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.
