WASHINGTON — The U.S. is expanding COVID-19 boosters as it battles the omicron surge, with the Food and Drug Administration allowing extra Pfizer shots for children as young as 12.

Boosters already are recommended for everyone 16 and older, and federal regulators on Monday decided they’re also warranted for 12- to 15-year-olds once enough time has passed since their last dose.

But the move, coming as classes restart after the holidays, isn’t the final step. The Centers for Disease Control and Prevention must decide whether to recommend boosters for the younger teens. Dr. Rochelle Walensky, the CDC’s director, is expected to rule later this week.

The FDA also said everyone 12 and older eligible for a booster can get one as early as five months after their last dose rather than six months.

Vaccines still offer strong protection against serious illness from any type of COVID-19. But health authorities are urging everyone who’s eligible to get a booster dose for their best chance at avoiding milder breakthrough infections from the highly contagious omicron mutant.

Children tend to suffer less serious illness from COVID-19 than adults. But child hospitalizations are rising during the omicron wave — most of them unvaccinated.

The vaccine made by Pfizer and its partner BioNTech is the only U.S. option for children of any age. About 13.5 million 12- to 17-year-olds — just over half that age group — have received two Pfizer shots, according to the CDC.