Vaccinations for the coronavirus continued on Christmas Day, as authorities worried that holiday gatherings will further spread infection and the federal government said anyone bound for the U.S. from the United Kingdom must first test negative for the virus.

Suzanne Czerniak, a diagnostic radiology resident at Yale New Haven Hospital in Connecticut, got her first coronavirus vaccine dose on Christmas afternoon.

She said she brought homemade cookies to the health-care staff working on the holiday to administer vaccinations, and the one who gave her a shot was sporting a holiday sweater.

“It’s the best Christmas present,” she said. “When I got the email that I was eligible and I needed to schedule the first shot, I cried. I cried when I got home, too.”

New York Gov. Andrew M. Cuomo (D) this week called on hospitals and nursing home operators to work over the holidays to vaccinate people.

In a Christmas message, Pope Francis called for equal distribution of coronavirus vaccines, “especially for the most vulnerable and needy,” across national boundaries. Rich countries have bought up billions of potential doses, which could leave some poor countries without sufficient supplies for years.

In Britain, Queen Elizabeth II’s televised Christmas remarks highlighted everyday acts of empathy and kindness. “Let the light of Christmas, the spirit of selflessness, love and, above all, hope, guide us in the times ahead,” she said.

On Friday, the Centers for Disease Control and Prevention signed an order that mandates American citizens and others flying from the United Kingdom must be tested for the coronavirus, a move U.S. officials hope will thwart a new faster-spreading variant of the virus.

The requirement, which takes effect Monday, could affect tens of thousands of travelers per month but stops short of an outright ban, as dozens of other nations, including Canada, have done.

The decision follows President Trump’s order in March barring entry to the United States by many foreign nationals who had been in the U.K. in the past 14 days. “This additional testing requirement will fortify our protection of the American public to improve their health and safety and ensure responsible international travel,” the CDC said in a statement.

Passengers must get a viral test — meaning one that detects current infections — within three days of their flight, the CDC said. Travelers are required to provide airlines written documentation of the results. PCR or antigen tests are both considered acceptable.

Officials in Washington took action after some state officials had loudly called for federal intervention.

In New York, Cuomo had said the United States should follow the lead of other countries and “halt travel until we know what we’re talking about and we know the facts.”

On Friday, he tweeted calling the CDC’s decision a “Christmas wish answered.”

Mark Jarrett, chief quality officer for New York’s Northwell Health, said the provider took a pause on vaccinations Christmas Day but will be “back in full swing,” starting on Saturday.

“We are going to be continuing vaccinations tomorrow, Sunday, all next week,” he said. “We did not do vaccines today, since it’s a holiday a lot of staff likes off. They’ve been working so hard and we do have resurgence going on. We felt that on both the human side and staffing side we wouldn’t do it, but we are back in full swing tomorrow.”

Jahan Fahimi, an emergency physician at UCSF Health in San Francisco, said the start of vaccinations have been a morale booster for weary health-care workers.

“Nobody is exhaling quite yet. Many of us gave gotten the first shot of the vaccine, we know we’re starting to develop a small amount of immunity, but nobody has enough immunity quite yet to feel completely at ease,” Fahimi said. “We’re still playing by the same set of rules, which is masking and all the PPE we buried ourselves under on a daily basis when we’re working.”

He urged people to continue to take precautions, stay home and avoid travel through the holidays, including New Year’s.

“I’m hopeful that if we can do the right thing now, we’ll be on the downslope sooner,” he said.

The Transportation Security Agency announced this week that it screened 1,191,123 individuals at airport checkpoints across the country on Wednesday, more than any day since March 16.

“A lot of us are nervous about that and what that’s going to mean,” said Stephen C. Dorner, an emergency physician at Massachusetts General Hospital. “But it’s going to take a little while to see what happens, since hospitalization rates lag behind infection rates.”

British officials have been alarmed at the swift spread of the new variant and are also concerned about an even faster-spreading mutation identified in South Africa. Researchers say there is no evidence either variant of the coronavirus is more deadly, and they are optimistic existing vaccines will combat them effectively. It is also possible the vaccines could quickly be updated if changes are needed, they said.

“Viruses constantly change through mutation, and preliminary analysis in the UK suggests that this new variant may be up to 70% more transmissible than previously circulating variants,” the CDC said.

Experts said the variants could already be working their way, undetected, through American communities, where coronavirus testing and the sequencing to track variants is less far-reaching than in many other countries.

The CDC notes on its website that a negative test result means a person was probably not infected when their sample was taken. But it could also mean “your sample was collected too early in your infection” and you could still become sick.

Given the uncontrolled spread of the coronavirus across the United States, there has been some debate among U.S. officials about whether banning flights made sense. Federal officials ultimately decided testing was the better approach. More than 329,000 people have died of the coronavirus in the United States.

Canada has banned flights from the U.K. until Jan. 6, “so we can prevent this new variant of covid-19 from spreading in Canada,” Prime Minister Justin Trudeau said.

According to the latest figures from the U.S. Bureau of Transportation Statistics and aviation data firm Cirium, more than 13,600 passengers flew directly from the U.K. to the United States in June.

That figure has continued to climb, and Airlines for America, the industry advocacy group, said total passengers arriving from London’s Heathrow Airport topped 30,000 in November.

Drew Harris, a population health analyst, said the new variant is a cause for caution, but much remains unknown.

Harris said the administration’s partial travel ban from March, which remains in effect, was “very porous.”

“There’s no sense of having a travel ban if you allow American citizens,” permanent residents and other exceptions, said Harris, who recently retired from Thomas Jefferson University in Philadelphia. He said testing is a useful step.

“If people are coming in, then it’s important they be tested to determine if they’re bringing the new strain in with them,” Harris said. “As bad as things are in the United States, they could get worse if we had a faster-spreading virus. But we don’t know that just yet.”

Benjamin Soloway contributed to this report.