At least some U.S. adults may get updated COVID-19 shots this fall, as government advisers voted Tuesday that it’s time to tweak booster doses to better match the most recent virus variants.

Advisers to the Food and Drug Administration wrestled with how to modify doses now when there’s no way to know how the rapidly mutating virus will evolve by fall — especially since people who get today’s recommended boosters remain strongly protected against COVID-19’s worst outcomes.

Ultimately the FDA panel voted 19-2 that COVID-19 boosters should contain some version of the super-contagious omicron variant, to be ready for an anticipated fall booster campaign.

“We are going to be behind the eight-ball if we wait longer,” said one adviser, Dr. Mark Sawyer of the University of California, San Diego.

The FDA will have to decide the exact recipe, but expect a combination shot that adds protection against either omicron or some of its newer relatives to the original vaccine.

“None of us has a crystal ball” to know the next threatening variant, said FDA vaccine chief Dr. Peter Marks. But “we may at least bring the immune system closer to being able to respond to what’s circulating” now rather than far older virus strains.

It’s not clear who would be offered a tweaked booster — they might be urged only for older adults or those at high risk from the virus. But the FDA is expected to decide on the recipe change within days and then Pfizer and Moderna will have to seek authorization for the appropriately updated doses, time for health authorities to settle on a fall strategy.

Current COVID-19 vaccines have saved millions of lives globally. With a booster dose, those used in the U.S. retain strong protection against hospitalization and death but their ability to block infection dropped markedly when omicron appeared. And the omicron mutant that caused the winter surge has been replaced by its genetically distinct relatives. The two newest omicron cousins, called BA.4 and BA.5, together now make up half of U.S. cases, according to the Centers for Disease Control and Prevention.