The U.S. Food and Drug Administration on Wednesday authorized COVID-19 boosters from Moderna and Johnson & Johnson, and also said Americans can choose a booster different from their original shot.
That paves the way for millions more Americans to get additional protection as the Delta variant sparks breakthrough infections in vaccinated people.
The FDA previously authorized booster shots of Pfizer's vaccine six months after inoculation for people age 65 and older and those at risk of severe disease or infection.
Last week, an advisory panel to the FDA voted to recommend Moderna's booster, half the strength of its initial shots, for the same groups.
The panel also recommended a second shot of the J&J vaccine for all recipients at least two months after receiving their first.
While data indicated that "mixing and matching" different boosters was safe, FDA officials are still unclear on which combination of shots is best.
Many countries including the UK have backed mix-and-match strategies for the widely used AstraZeneca vaccine, which is not authorized in the U.S. but based on similar technology as J&J's.
FDA officials suggested last week they were considering lowering the recommended age for Pfizer's boosters to as young as 40 based on data from Israel, where they have already been widely used.
They did not do so on Wednesday, but said they planned to update the public in the coming weeks.
Meanwhile, a CDC advisory committee on Thursday will make its own recommendations about which groups of people should get the Moderna and J&J boosters, which the agency's director will use to inform her final decision.
Over 11 million people have received a booster dose so far, according to data from the CDC.