Advisers to the U.S. Food and Drug Administration on Thursday will discuss and vote on whether to recommend targeting one of the currently dominant XBB coronavirus variants in updated COVID-19 shots being developed for a fall vaccination campaign.

FDA staff reviewers in documents released this week said available evidence suggests this year's shots should target an XBB variant. XBB and its offshoots, which now account for most U.S. infections, are descendents of the Omicron variant that caused COVID cases to surge to record levels early last year.

U.S. regulators are looking to bring the next COVID-19 shots more closely in line with the circulating virus.

The next generation shots should select a single XBB-related target, the FDA's staff reviewers suggested. A so-called monovalent vaccine would be a divergence from the most recent bivalent COVID boosters that targeted both the original strain of the coronavirus and Omicron.

The expert panel meeting comes after an advisory group to the World Health Organization (WHO) last month recommended the next wave of COVID-19 booster shots be updated to target XBB subvariants. Europe's medicine regulators backed that recommendation.

The FDA will take recommendations from its outside experts into consideration before making a final decision on composition of the shots.

The panel will discuss, but not vote on, exactly which subvariant it believes the vaccines should target, with XBB.1.5, XBB.1.16 and XBB.2.3 among the candidates.

COVID vaccine makers Pfizer/BioNtech, Moderna and Novavax are already developing versions of their respective vaccines targeting XBB.1.5 and other currently circulating subvariants.

Only about 17% of the U.S. population - some 56.5 million people - received a COVID booster during the 2022-2023 vaccination season, according to government data through early May. Older Americans were boosted at a higher rate than the general population.

"The highest risk people are really older adults as well as people with significant comorbidities," said Dr. Paul Sax from Brigham and Women's Hospital in Boston. "That's the primary group to target."

Between Moderna and Pfizer, Morningstar analyst Karen Andersen expects 75 million doses to be sold in the United States during the 2023-2024 campaign.

Moderna last month said it continues to expect the U.S. annual COVID-19 market to be 100 million doses, largely in line with Pfizer's view of 102 million.

COVID deaths and hospitalizations have declined in 2023, and the U.S. government in May ended its COVID Public Health Emergency that had allowed millions of Americans receive vaccines, tests and treatments at no cost.

"The good news is that we haven't had any surges in some time," Sax said. "The less good news is that there's still COVID circulating and people still get it sort of year-round."

(Reporting by Bhanvi Satija and Sriparna Roy in Bengaluru; Editing by Michael Erman and Bill Berkrot)