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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 that 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.