The U.S. Food and Drug Administration cleared a broad path for millions more Americans to receive COVID-19 vaccine booster shots, as the nation looks to bolster its defenses and prevent another virus surge.

The agency said in a statement Wednesday that Moderna Inc. vaccine recipients 65 and over can receive a third shot, as can adults 18 and up who are at high risk of severe COVID or with frequent institutional or occupational exposure to the virus that causes the disease.

All J&J recipients 18 and older are also eligible for a booster shot at least two months after receiving their first dose.

The agency also allowed each of the available COVID vaccines to be used as a booster dose for eligible individuals following completion of a primary vaccination.

The moves will mean the U.S. has a bigger toolkit to try to limit a potential winter virus rebound. The summer’s delta-variant fueled spike in infections helped increase urgency to make boosters available, and health officials across the U.S. are eager to forestall a rebound in cases that could cripple hospitals and disrupt work and school this winter.

The clearances came after a panel of expert advisers to the FDA unanimously backed the Moderna and J&J booster regimens in two days of meetings last week. Regulators have now signed off on boosters for all three coronavirus vaccines available in the U.S.

Last month, the FDA said people 65 and over and others who are at heightened risk of severe COVID were eligible for a booster dose of the vaccine developed by Pfizer Inc. and BioNTech SE.

Moderna shares climbed 1.9 percent in after-hours trading in New York, while J&J shares gained 0.4 percent and Pfizer shares rose 0.2 percent. U.S.-traded shares of Germany-based BioNTech gained 1.3 percent.

The Moderna booster shot authorized by the FDA is half the dose that is given in the initial two-shot series, and it should be given at least six months after the initial inoculation, regulators said.

The FDA said that a single booster dose of the Pfizer vaccine may be given at least six months after completing the primary series to people 18 to 64 with frequent institutional or occupational exposure to the coronavirus.

In permitting mixing and matching, the FDA is allowing J&J vaccine recipients to receive an additional dose of any cleared vaccine after two months. Likewise, recipients of Moderna and Pfizer who are eligible for a booster would receive their booster, including J&J’s shot, at least six months after their initial immunization regimen.

Seen as a convenient, effective alternative to two-shot messenger RNA vaccines, J&J’s single-shot immunization has seen far less use in the U.S., in part because it isn’t as effective. The drugmaker has also experienced manufacturing problems that limited the shot’s distribution.

Before the Moderna and J&J booster shots can be administered, the Centers for Disease Control and Prevention’s Advisory Panel on Immunization Practices will make further recommendations about who should receive them. The panel is scheduled to discuss boosters Thursday.

The next big milestone for the U.S. immunization effort looms next week, when the FDA advisory panel is expected to weigh Pfizer’s proposed COVID vaccine for children ages 5 to 11. If authorized, it could begin to roll out to pediatricians’ offices and drugstores as soon as next month.