U.S. Food and Drug Administration staff on Monday said updated Covid boosters should target XBB omicron subvariants for the upcoming fall and winter vaccination campaign. 

The U.S. should use a monovalent vaccine targeting either XBB.1.5, XBB.1.16, or XBB.2.3, collectively the dominant strains nationwide, the staff said in a briefing document. 

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The FDA staff made the conclusion ahead of a meeting on Thursday, when a panel of external advisors to the agency will recommend a strain for new Covid shots to target later this year. There is no set date for when the vaccination campaign will begin.

Vaccine manufacturers will be expected to update their shots once that strain is selected.

Pfizer, Moderna and Novavax are already developing versions of their respective vaccines targeting XBB.1.5 and other circulating variants.

The upcoming strain selection will be crucial to those companies’ abilities to compete in the fall, when the U.S. is expected to shift vaccine distribution to the private sector. That means all three companies will start selling their updated Covid shots directly to health-care providers.

The FDA staff’s decision comes weeks after an advisory group to the World Health Organization recommended that Covid booster shots target XBB variants. 

Scientists have said XBB strains are some of the most immune-evasive subvariants to date.

Those strains accounted for more than 95% of Covid cases in the U.S. as of early June, according to the FDA staff. 

They noted that the proportion of XBB.1.5 cases is declining, but both XBB.1.16 and XBB.2.3 are “on the rise.” 

Last year’s Covid boosters were bivalent, meaning they targeted the original strain of the virus and omicron variants BA.4 and BA.5. Those variants dominated cases nationwide last fall and winter. 

Uptake has been sluggish. Only about 17% of the U.S. population has gotten Pfizer’s and Moderna’s bivalent boosters since they were approved in September, according to the Centers for Disease Control and Prevention.