U.S. health advisers on Thursday endorsed new COVID-19 boosters that target today’s most common omicron strains, saying if enough people roll up their sleeves, the updated shots could blunt a winter surge.

The tweaked shots made by Pfizer and rival Moderna promise Americans a chance at their most up-to-date protection at yet another critical period in the pandemic. They’re combination or “bivalent” shots, half the original vaccine and half protection against the BA.4 and BA.5 omicron versions now causing nearly all COVID-19 infections.

Advisers to the Centers for Disease Control and Prevention struggled with who should get the new booster and when — because only a similarly tweaked vaccine, not the exact recipe, has been studied in people so far.

But ultimately the panel deemed it the best option considering the U.S. still is experiencing tens of thousands of COVID-19 cases and about 500 deaths every day — even before an expected new winter wave.

“I think they’re going to be an effective tool for disease prevention this fall and into the winter,” said CDC adviser Dr. Matthew Daley of Kaiser Permanente Colorado.

Comparing the tweak that has been studied in people and the one the U.S. actually will use, “it is the same scaffolding, part of the same roof, we’re just putting in some dormers and windows,” said Dr. Sarah Long of Drexel University.

The CDC is expected to adopt that recommendation soon, the last step before shots can begin. Millions of doses are expected to reach vaccination sites nationwide by Labor Day, CDC officials said.

The original COVID-19 vaccines still offer strong protection against severe illness and death, especially among younger and healthier people who’ve gotten at least one booster.

But those vaccines were designed to target the virus strain that circulated in early 2020. Effectiveness drops as new mutants emerge and the longer it’s been since someone’s last shot. Since April, hospitalization rates in people over age 65 have jumped, the CDC said.

The new updated shots are only for use as a booster, not for someone’s first-ever vaccinations. The Food and Drug Administration cleared Pfizer’s bivalent option for people 12 and older while Moderna’s is for adults only.

A big unknown: Exactly how much benefit people will get from one of those extra shots.

The CDC said more than 1,400 people have been included in studies of a prior tweak to the vaccine recipe — targeting an earlier omicron strain named BA.1. That omicron-targeting combo shot proved safe and able to rev up virus-fighting antibodies — and European regulators on Thursday recommended using that type of booster.

In the U.S., the Food and Drug Administration wanted fall boosters to target the currently circulating omicron strains — and rather than waiting until possibly November for more human studies to be finished, the agency accepted mouse testing that showed the newer tweak sparked a similarly good immune response.

That’s how flu vaccines are updated every year, the CDC noted.

Still, several CDC advisers said that to get the maximum benefit, people will need to wait longer between their last vaccination and getting the new booster than the two months that the FDA set as the minimum. Waiting at least three months would be better, from the last shot or if someone had recently recovered from COVID-19, they said.

Before this new COVID-19 booster update, people 50 and older already were urged to get a second booster of the original vaccine — and those who did saw some extra protection especially the longer it had been since their last shot, said CDC’s Dr. Ruth Link-Gelles.

The new combination booster “should provide at least similar or better protection against omicron since it’ll be a better match” to today’s virus strains, she told the panel.