WASHINGTON — U.S. health advisers announced 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.