The U.S. is expanding COVID-19 boosters, ruling that 16- and 17-year-olds can get a third dose of Pfizer's vaccine.

The U.S. and many other nations already were urging adults to get booster shots to pump up immunity that can wane months after vaccination, calls that intensified with the discovery of the worrisome new omicron variant.

On Thursday, the Food and Drug Administration gave emergency authorization for 16- and 17-year-olds to get a third dose of the vaccine made by Pfizer and its partner BioNTech -- if it's been six months since their last shot.

There's one more step: The Centers for Disease Control and Prevention must formally recommend the boosters for this age group and a decision is expected soon.

The Pfizer vaccine is the only option in the U.S. for anyone younger than 18, either for initial vaccination or for use as a booster. It's not yet clear if or when teens younger than 16 might need a third Pfizer dose.

Vaccinations for children as young as 5 just began last month, using special low-dose Pfizer shots. By this week, about 5 million 5- to 11-year-olds had gotten a first dose.

The extra-contagious delta variant is causing nearly all COVID-19 infections in the U.S., and in much of the world. It's not yet clear how vaccines will hold up against the new and markedly different omicron mutant. But there's strong evidence that boosters offer a jump in protection against delta-caused infections, currently the biggest threat.

Complicating the decision to extend boosters to 16- and 17-year-olds is that the Pfizer shot -- and a similar vaccine made by Moderna -- have been linked to a rare side effect. Called myocarditis, it's a type of heart inflammation seen mostly in younger men and teen boys.

Health officials in Israel, which already gives boosters to teens, have said the side effect continues to be rare with third doses.

A U.S. study this week offered additional reassurance. Researchers from children's hospitals around the country checked medical records and found the rare side effect usually is mild and people recover quickly, while COVID-19 itself can cause more serious heart inflammation. The research was published Monday in the journal Circulation.

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