WASHINGTON (AP) - U.S. regulators on Wednesday signed off on extending COVID-19 boosters to Americans who got the Moderna or Johnson & Johnson vaccine and said anyone eligible for an extra dose can get a brand different from the one they received initially.

The Food and Drug Administration’s decisions mark a big step toward expanding the U.S. booster campaign, which began with extra doses of the Pfizer vaccine last month. But before more people roll up their sleeves, the Centers for Disease Control and Prevention will consult an expert panel Thursday before finalizing official recommendations for who should get boosters and when.

The latest moves would expand by tens of millions the number of Americans eligible for boosters and formally allow “mixing and matching” of shots - making it simpler to get another dose, especially for people who had a side effect from one brand but still want the proven protection of vaccination.

Specifically, the FDA authorized a third Moderna shot for seniors and others at high risk from COVID-19 because of their health problems, jobs or living conditions - six months after their last shot. One big change: Moderna’s booster will be half the dose that’s used for the first two shots, based on company data showing that was plenty to rev up immunity again.

For J&J’s single-shot vaccine, the FDA said all U.S. recipients, no matter their age, could get a second dose at least two months following their initial vaccination.

As for mixing and matching, the FDA said it’s OK to use any brand for the booster regardless of which vaccination people got first. The interchangeability of the shots is expected to speed the booster campaign, particularly in nursing homes and other institutional settings where residents have received different shots over time.

The agency’s mix-and-match decision was based on preliminary results from a government study of different booster combinations that showed an extra dose of any type revs up levels of virus-fighting antibodies. That study also showed recipients of the single-dose J&J vaccination had a far bigger response if they got a full-strength Moderna booster or a Pfizer booster rather than a second J&J shot. The study didn’t test the half-dose Moderna booster.

Health authorities stress that the priority still is getting first shots to about 65 million eligible Americans who remain unvaccinated.