WASHINGTON - As President Joe Biden exhorts Americans to get coronavirus vaccines and booster shots to strengthen protections against the delta and omicron variants, another age group might soon become eligible for the boosters: 16- and 17-year-olds. 

Pfizer and its partner BioNTech are expected to ask the Food and Drug Administration in the coming days to authorize its booster shot for that age group, according to two people familiar with the situation. The regulators are expected to sign off quickly, said the individuals, who spoke on the condition of anonymity because they were not authorized to discuss the issue.

Currently, Americans who are 18 and older are eligible for boosters six months after receiving the second shot of the Pfizer-BioNTech or Moderna vaccines. They can receive the Johnson & Johnson booster two months after getting the single-shot vaccine.

The move to expand eligibility among teens comes as the White House is grappling with the omicron variant that has captured the world's attention. With information on the variant scant and anxieties high, Biden urged Americans on Monday to get vaccinated and schedule boosters, calling the shots "the best protection against this new variant or any of the variants out there."

Omicron has not yet been found in the United States, though officials say it is only a matter of time before it is detected. The variant has been identified in Canada, Britain, South Africa and Israel. Scrambling to respond, the United States has imposed travel restrictions on eight countries in southern Africa, vaccine makers are rushing to test their formulas' effectiveness, and scientists around the globe are trading information to try to determine whether omicron is more transmissible and virulent than the delta variant.

"Given the current overall situation of the pandemic, FDA will evaluate any such [emergency use authorization] request in a very timely manner," the agency said.

Kit Longley, a Pfizer spokesman, said: "We expect to share updates on this soon."

The Pfizer-BioNTech and Moderna vaccines have been linked to a rare side effect called myocarditis, a swelling of the heart muscle, in older male adolescents and young men. That has prompted regulators and some government advisers to take a cautious approach to boosters for younger people, though experts have repeatedly said the risks of heart problems from the coronavirus itself is far higher than from vaccination.

Recent data from Israel on booster shots has provided further reassurance to federal regulators about authorizing extra doses for 16- and 17-year-olds, said one of the people familiar with the situation. Those studies have shown that the side effect is uncommon and that the cases that do occur tend to be mild.

Jeanne Marrazzo, an infectious-disease physician at the University of Alabama at Birmingham, said she supports extending boosters to 16- and 17-year-olds. She said anything the United States can do to increase protections "before omicron hits can potentially help and certainly won't hurt."

Eric Topol, a professor of molecular medicine at Scripps Research, also endorsed the change, saying it would restore the vaccine to 95% effectiveness in adolescents whose protection has waned for months.

"This is a three-shot vaccine for all, as we've clearly learned," he said. "Not just for the protection of the vaccines but also to blunt transmission potential."

Still, some experts expressed caution.

"While I am very supportive of adults receiving boosters as the data on benefit are clear, the risk-versus-benefit of booster vaccines for adolescents is less clear," said David Boulware, an infectious-disease expert at the University of Minnesota Medical School. "Certainly, large-scale population data from Israel will be informative as to the safety of booster doses in adolescents. Overall, boosters in adolescents would be most important in households where there are immunocompromised adults or contact with elderly adults."

Federal regulators in recent months have sought to expand the number of people who are eligible for boosters and make the guidelines more uniform. Last month, the FDA authorized Pfizer-BioNTech and Moderna boosters for people 18 and older. Earlier, the agency cleared the Johnson & Johnson booster for people 18 and older.