The Food and Drug Administration on Monday authorized booster shots of the Pfizer-BioNTech coronavirus vaccine for 12-to-15-year-olds, part of a broader effort to expand protection as schools, airlines and businesses struggle with massive disruptions caused by a surging omicron variant.

The FDA also shortened to five months the time required between the second dose of the Pfizer-BioNTech vaccine and a booster shot. Previously, the interval was six months. In addition, it cleared third vaccine doses for some younger children with impaired immune systems. The moves are scheduled to be reviewed by the Centers for Disease Control and Prevention and its advisers this week, with CDC Director Rochelle Walensky expected to sign off as soon as Wednesday.

The changes came as the United States averaged more than 440,000 new coronavirus cases each day as of Monday, double the previous week, according to seven-day averages tracked by The Washington Post. The single-day total for Monday edged above 1 million cases, but most states reported data for multiple days because of the holiday weekend.

The number of patients hospitalized with confirmed or suspected covid-19, the disease caused by the coronavirus, is up 31 percent from last week, with about 1,500 Americans dying each day, a 50 percent increase since Dec. 1.

The sharp rise in cases, and the resulting tumult, ignited debates on how to respond. Florida Gov. Ron DeSantis (R) on Monday rejected calls to open state-run coronavirus testing sites and instead said he would order more monoclonal-antibody treatment centers to open. He said the state is responding “rationally” by emphasizing treatments and not closing schools or issuing mask and vaccine mandates.

“When you look at what’s going on in other states, they’re letting hysteria drive them to doing really damaging things,” DeSantis said at a news conference in Fort Lauderdale, Fla.

But others took far different tacks, calling for tougher safety measures involving vaccines, tests and masks. Starbucks told its nearly 220,000 U.S. employees that they must be fully vaccinated against the virus or submit to weekly testing. The workers must notify the company of their vaccination status by Jan. 10.

Congress’s attending physician warned lawmakers and their staff members that the seven-day test-positivity rate at the U.S. Capitol increased from less than 1 percent to more than 13 percent. Brian P. Monahan urged as many people as possible to work remotely and to wear better masks — a KN95 mask or N95 mask — to improve protection against the highly contagious omicron variant.

“The daily case rates will increase even more substantially in the coming weeks,” Monahan wrote. “These cases represent both a rising rate of ‘breakthrough infections’ among mostly previously vaccinated individuals and several unvaccinated individuals.”

Meanwhile, about 3,000 flights were canceled Monday, as snowstorms led to widespread delays along the East Coast and further strained a system already stressed by coronavirus-induced staffing shortages.

The nation’s school systems also faced a raft of difficult decisions, with some deciding to revert, at least temporarily, to remote teaching. Many large districts, however, are forging ahead with a resumption of in-person learning — although some are imposing stricter rules involving testing and masks.

Public schools in the District, which are scheduled to reopen later this week, are requiring proof of a negative test result for all returning students and teachers. Other school systems, including in Los Angeles County, are beefing up mask rules.

Omicron is the fifth coronavirus variant of concern and is spreading rapidly around the world. Here’s what we know. (Luis Velarde/The Washington Post)

For increasingly anxious parents, the FDA authorization of Pfizer-BioNTech booster shots for 12-to-15-year-olds came as a relief. Some health experts lauded the move, but others criticized it, saying the initial two-dose series of shots provided adequate protection against severe disease. They questioned whether healthy children should be given an extra shot of a vaccine that has been linked to a rare cardiac side effect called myocarditis.

On a call with reporters explaining their decisions, FDA officials said the agency broadened access to boosters because data from real-world experience and laboratory tests indicates the shots significantly strengthen protection — especially against the omicron variant — while posing minimal risk.

The officials cited Israeli data involving more than 6,300 12-to-15-year-olds that showed no new safety issues among those who received a third dose of the Pfizer-BioNTech vaccine. And they said there were no new cases of heart-related complications.

Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said the risk of myocarditis, rare in any case, was lower after a booster shot than after the second shot. Even when myocarditis has occurred after second shots, he said, the cases have almost all been mild.

He said new data — and the inferences the agency drew from the information — suggests “the risk of myocarditis with third doses in the 12-to-15-year age range is likely to be quite acceptable given the potential benefits” of preventing serious illnesses and hospitalizations.

The agency also relied on data from Israel in shortening the interval between second and third doses of the vaccine for adolescents and adults. Scientists reported recently that no new safety concerns arose among more than 4 million people who received boosters at least five months after getting the second shot of the Pfizer-BioNTech vaccine.

The shorter interval means someone who got two shots of the Pfizer-BioNTech vaccine can get a third shot of that vaccine or another one after five months, rather than six months. Marks said the interval was changed for the Pfizer-BioNTech vaccine because there was data to support such a move.

The intervals for the other vaccines are unchanged. People who got the initial two-shot Moderna series are supposed to wait at least six months to get a booster, and those who received the Johnson & Johnson vaccine are supposed to wait two months. Marks said the FDA would be receptive to considering shortening the Moderna interval if provided with the appropriate data.

Regarding immunocompromised children, the FDA said it authorized a third dose for 5-to-11-year-olds who have undergone organ transplants or have impaired immune systems for other reasons because they may not mount an adequate response to the initial vaccine series. The agency previously cleared a third dose for immunocompromised individuals 12 years and older. The doses can be administered 28 days after the second shot, the agency said.

The FDA said 5-to-11-year-olds who are not immunocompromised do not need boosters at this time — they were cleared for initial shots just recently, in late October. But the officials said they are closely watching that age group to see if they will need boosters.

“We certainly understand the pressing need in that age range,” Marks said, adding that the agency will move “with the appropriate speed” if boosters are necessary. Until then, he said, it is critical for parents to get young children vaccinated, noting that only about 25 percent in that age group have been vaccinated.

For months, public health officials have urged eligible individuals to get booster shots, pointing to data suggesting that vaccine-induced immunity wanes. Only about one-third of eligible Americans have received a booster shot, according to the CDC.

Even as some data suggests that the omicron variant may be less dangerous than the delta variant, the number of children being hospitalized across the country has crept up in recent weeks.

Asked by reporters whether the FDA was considering a vaccine that specifically targeted the omicron variant, the FDA officials said they did not know whether that will be needed. It isn’t clear yet whether the omicron surge will be a quick wave or persist.

“The last thing we want to do is just be going through, and doing this and that, if the current boosters will really do the job against omicron,” said acting FDA commissioner Janet Woodcock.

Vaccine manufacturers are developing and testing omicron-specific shots in case they are needed, the officials said.

Lori Aratani, Jacob Bogage, Lori Rozsa, Brittany Shammas and Amy B Wang contributed to this report.