By LAURAN NEERGAARD and MATTHEW PERRONE, Associated Press

U.S. health advisers said Thursday that some Americans who received Moderna’s COVID-19 vaccine should get a half-dose booster to bolster protection against the virus.

The panel of advisers to the Food and Drug Administration voted unanimously to recommend a booster shot for seniors, adults with other health problems, jobs or living situations that put them at increased risk for COVID-19.

The recommendation is non-binding, but it’s a key step toward expanding the U.S. booster campaign to millions more Americans. Many people who got their initial Pfizer shots at least six months ago are already getting a booster after the FDA authorized their use last month.

As for the dose, initial Moderna vaccination consists of two 100-microgram shots. But Moderna says a single 50-microgram shot should be enough for a booster.

The agency convened its experts Thursday and Friday to weigh in on who should get boosters and when for people who received the Moderna and Johnson & Johnson shots earlier this year.

The FDA will use its advisers’ recommendations in making final decisions for boosters from both companies. Assuming a positive decision, there’s still another hurdle: Next week, a panel convened by the Centers for Disease Control and Prevention will offer more specifics on who should get one.

U.S. officials stress the priority is to get shots to the 66 million unvaccinated Americans who are eligible for immunization — those most at risk as the extra-contagious delta variant of the coronavirus has burned across the country.

“It’s important to remember that the vaccines still provide strong protection against serious outcomes” such as hospitalization and death from COVID-19, said FDA vaccine chief Dr. Peter Marks.

But Marks said it’s also become clear there is some waning of protection against milder infections with all three of the coronavirus vaccines used in the U.S. And he encouraged the advisory panel to consider if the evidence backs similar booster recommendations for all of them as well, because that would “create the least confusion” for the public.

Moderna is seeking FDA clearance for a booster used just like Pfizer’s: For people 65 and older, or adults with other health problems, jobs or living situations that put them at increased risk of serious coronavirus — once they are at least six months past their last dose.

What’s the evidence that the Moderna vaccine’s protection is waning? As the delta variant surged in July and August, people who were more recently vaccinated had a 36% lower rate of “breakthrough” infections compared with those vaccinated longer ago, Moderna’s Dr. Jacqueline Miller told FDA’s advisers.

A study of 344 people found the booster shot six months after initial vaccinations restored virus-fighting antibodies to levels seen after recipients’ last dose — and that included large jumps in antibodies able to target the delta variant, Miller said.

One very rare side effect of both the Moderna and Pfizer vaccines is heart inflammation, particularly among young men soon after the second dose — and one lingering question is whether another dose could spark more cases. Moderna’s booster study wasn’t large enough to spot such a rare risk.

But Israel began offering Pfizer boosters sooner than the U.S. and to more of its population. Thursday, Dr. Sharon Alroy-Preis of Israel’s health ministry told the FDA panel that after 3.7 million booster doses administered, there’s no sign the extra shot is any riskier — despite an intense investigation into heart inflammation.