June 17, 2024

WASHINGTON — The Food and Drug Administration on Thursday said it had advised coronavirus vaccine manufacturers to update their booster formulations to target the fast-spreading Omicron subvariants known as BA.4 and BA.5, which are now estimated to account for more than half of virus cases in the United States.

The decision came just two days after the agency’s committee of independent vaccine experts overwhelmingly voted for regulators to adopt more advanced vaccines that would incorporate forms of Omicron, an acknowledgment that the current shots may no longer be adequately protective for at least some Americans by the time a possible fall or winter surge arrives.

“As we move into the fall and winter, it is critical that we have safe and effective vaccine boosters that can provide protection against circulating and emerging variants,” Dr. Peter Marks, who oversees the F.D.A.’s vaccines office, said in a statement Thursday.

Dr. Marks said the agency was not asking manufacturers to change the formulation for initial doses of their coronavirus vaccines, since the current makeup “provides a base of protection against serious outcomes.”

“We expect this coming year to be a transitional period when this modified booster vaccine may be introduced,” he said.

The choice to take aim at the subvariants, arguably the most transmissible coronavirus variants to date, was clouded by considerable uncertainty around what form the virus may take later this year, making the design of Covid-19 shots uneasy guesswork for Pfizer-BioNTech and Moderna, the two main manufacturers, and the federal scientists charged with guiding them on how to tailor their products.

Pfizer and Moderna are expected to take up the F.D.A. recommendation and start producing reformulated doses this summer, looking to have them ready by fall if federal regulators authorize them for a new booster campaign. Pfizer said on Tuesday that a shot aimed at the chosen subvariants could be ready for use in early October, while Moderna has projected availability in late October or early November.

Sharon LaFraniere contributed reporting.

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