New York City has confirmed its first two cases of the new, more infectious U.K. variant of the novel coronavirus, Mayor Bill de Blasio said Wednesday morning at a press conference, posing a threat to the city’s already increasingly burdened hospital system. 

The two cases were diagnosed in late December, and were only confirmed via genetic testing to be caused by the variant Wednesday, Dr. Dave Chokshi, the city’s health commissioner, said at the press conference, raising the likelihood that the variant is already spreading widely through the city. 

Even before New York City reported its first case of the novel coronavirus in March 2020, there were already thousands of cases of COVID-19 in the city, according to models released early in the pandemic by health researchers. 

The Wadsworth Center, New York’s public health laboratory, has processed 1,600 samples of coronavirus to test if they are the U.K. variant, according to Jonah Bruno, a spokesperson for the state Department of Health. Most of the samples, sent from labs and hospitals around the state, are selected at random, except for those from contacts of people who have been confirmed as having the U.K. variant. Those samples are currently being sequenced, Bruno said. The department did not respond to a question about why it took two weeks to get the test results for the samples from the two New York City residents who were found to carry the U.K. variant.

One person who had the U.K. variant of the virus is a resident of Manhattan, and the other lives in Queens, Chokshi said. He added that the city’s test-and-trace team had already reached out to contacts of the two people. 

There are currently 15 confirmed cases of the variant statewide, including the two in the city. 

De Blasio said he was “very concerned” about the spread of the variant in the city, and urged the federal government to adopt a travel ban from the U.K. One of the people who was found to have the variant had recently returned from traveling to the U.K., he said. 

“Anyone who's older, anyone who has preexisting conditions, should really limit their movements, should only do the most essential things,” he said. “If you're one of the folks who's vulnerable, don't take chances.”

The variant, which is spreading around the world, has already been found in ten other states. 

Studies have shown that the variant is more infectious than the other COVID-19 strains circulating in the U.S., though it does not cause a more severe case or increase likelihood of death. 

However, the variant has led to a surge in coronavirus cases in the U.K., overwhelming hospitals. 

A state dashboard shows that 77% of New York City’s hospital beds are occupied, a number that has been increasing since the beginning of the year. Only a quarter of city ICU beds are available.

The city saw 279 hospital admissions on Jan. 11, De Blasio said Wednesday, the highest number in a week. 

Throughout the state, hospital admissions rates have been increasing steadily for two weeks, now at 4.62 per 100,000 people. The percentage of New York City residents who have tested positive for the coronavirus has been dropping slightly since the New Year, to 8.39% on Jan. 11.