Washington (AP) — The U.S. on Wednesday authorized its first update to COVID-19 vaccines, booster doses that target today’s most common omicron strain. Shots could begin within days.

The move by the Food and Drug Administration tweaks the recipe of shots made by Pfizer and rival Moderna that already have saved millions of lives. The hope is that the modified boosters will blunt yet another winter surge — and help tamp down the BA.5 omicron relative that continues to spread.

Until now, COVID-19 vaccines have targeted the original coronavirus strain, even as wildly different mutants emerged. The new U.S. boosters are combination, or “bivalent,” shots. They contain half that original vaccine recipe and half protection against the newest omicron versions, BA.4 and BA.5, that are considered the most contagious yet.

The combination aims to increase cross-protection against multiple variants.

“It really provides the broadest opportunity for protection,” Pfizer vaccine chief Annaliesa Anderson told The Associated Press.

The updated boosters are only for people who have already had their primary vaccinations, using the original vaccines. Doses made by Pfizer and its partner BioNTech are for anyone 12 and older while Moderna’s updated shots are for adults — if it has been at least two months since their last primary vaccination or their latest booster. They’re not to be used for initial vaccinations.

There’s one more step before a fall booster campaign begins: The Centers for Disease Control and Prevention must recommend who should get the additional shot. An influential CDC advisory panel will debate the evidence Thursday — including whether people at high risk from COVID-19 should go first.