The U.S. is expanding COVID-19 boosters, opening a third dose of Pfizer's vaccine to people as young as age 16.

The U.S. and many other nations already urge adults to get a booster to pump up their immunity months after initial vaccination.

In the U.S., the Pfizer vaccine is the only option for children and teenagers.

The Food and Drug Administration on Thursday cleared an extra dose for those ages 16 and 17, and the Centers for Disease Control and Prevention rapidly endorsed a booster for those ages.

"Today, CDC is strengthening its booster recommendations and encouraging everyone 16 and older to receive a booster shot," CDC Director Dr. Rochelle Walensky said in a press release. "Although we don’t have all the answers on the Omicron variant, initial data suggests that COVID-19 boosters help broaden and strengthen the protection against Omicron and other variants."

The CDC recommends that the booster be received six months after their second COVID vaccine shot.

It's not clear if younger teens will need one.