US Food and Drug Administration staff on Monday said the updated Covid boosters should aim Omicron subvariants of XBB for the upcoming autumn and winter vaccination campaign.

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

FDA staff made the conclusion ahead of a meeting Thursday, when a panel of outside advisers to the agency will recommend new Covid shots be targeted later this year. There is no set date when the vaccination campaign will start.

Vaccine manufacturers are expected to update their shots once this strain is selected.

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

The upcoming strain selection will be critical to the competitiveness of these companies in the fall, when the U.S. is expected to shift vaccine distribution to the private sector. This means that all three companies will start selling their updates Covid footage directly to healthcare providers.

The FDA staff’s decision comes weeks after a World Health Organization advisory group recommended that Covid boosters target XBB variants.

The XBB strains are among the most immune-evasive subvariants to date, the researchers said.

These tribes represented more than 95% of Covid cases in the U.S. since early June, according to FDA officials.

They noted that the proportion of XBB.1.5 cases is decreasing, 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 the omicron variants BA.4 and BA.5. These variants dominated the national cases last fall and winter.

The reception was slow. Just about 17% The US population has received bivalent boosters from Pfizer and Moderna since approval in September, according to the Centers for Disease Control and Prevention.