WASHINGTON — The Food and Drug Administration on Wednesday authorized booster shots for tens of millions of recipients of Moderna’s two-dose coronavirus vaccine and the roughly 15 million who got Johnson & Johnson’s single-dose shot, significantly expanding the nation’s campaign to bolster protection for vulnerable people.

The agency also updated its authorizations for all three vaccines — Moderna, Johnson & Johnson and Pfizer-BioNTech — to allow medical providers to boost people with a different Covid-19 vaccine than the one they initially received, a strategy known as “mix and match.”

That update may dampen interest in Johnson & Johnson’s vaccine, which studies have found provides less protection than the other two. Providers will have the option of offering a Moderna or Pfizer-BioNTech booster, which could trigger a more substantial jump in protective antibodies.

The regulators, who already authorized boosters for high-risk Pfizer-BioNTech recipients last month, did not recommend any one vaccine over another as a booster. They also remained silent on whether it is preferable to stick with the same vaccine when possible.

The latest authorizations arrived in the middle of a busy stretch of regulatory decisions on vaccines. The F.D.A. is expected in the coming weeks to decide on whether Pfizer-BioNTech’s vaccine should be authorized for children ages 5 to 11. The agency’s independent vaccine committee is set to vote Tuesday to consider the matter and make a recommendation.

Regulators may also decide as early as November whether to authorize boosters for younger Americans who are not at high risk. While many vaccine experts have questioned whether younger, healthy people need booster shots, some advisory committee members last week urged the agency to move to expand eligibility.

The F.D.A.’s move to make a much larger swath of Americans eligible for additional shots on Wednesday followed unanimous votes last week from a key advisory committee to recommend the authorizations. A separate committee advising the Centers for Disease Control and Prevention is scheduled to vote Thursday on its own recommendations for Moderna and Johnson & Johnson boosters.

The C.D.C. director typically endorses that panel’s recommendations as a final step before vaccines are put into use. If the agency agrees with the F.D.A.’s decision, booster shots could be offered as soon as this weekend.

Eligible Moderna recipients could get a half-dose booster of that vaccine at least six months after their second dose. Like Pfizer recipients, the eligible would include people 65 and older, and younger adults at high risk of severe Covid-19 because of medical conditions or where they work. Johnson & Johnson recipients — all of whom are 18 and older — will be eligible for a second shot at least two months after the first.