NEW YORK — The explosion of COVID-19 cases in the New York City area resulted largely from infected patients who flew in from Europe, genome scientists say.

Researchers at NYU Langone Health said they’ve analyzed 75 samples from patients who were diagnosed with COVID-19, the disease caused by the novel coronavirus, at New York-area hospitals last month.

About two-thirds of the samples appear to have European origins, said Adriana Heguy, director of the Genome Technology Center at the medical center. The virus appears to have been imported to New York from the U.K. and several European countries, including France, Austria and the Netherlands, she said.

Genome sequencers are able to roughly correlate how a virus is spreading around the world by examining small mutations in a gene sequence of the pathogen as it is transmitted from person to person. In the case of the coronavirus, whose RNA consists of about 30,000 genetic bases, or letters, it mutates about twice a month.

Those minor mutations tend not to change the potency of the virus. But they provide clues for genetic detectives to chart how they shift subtly over time, allowing them to create sprawling family trees, or phylogenies, that show how the coronavirus has spread from one part of the world or country to the next.

One of the first cases Heguy’s team sequenced, collected in early March, came from a Long Island resident with no travel history whose viral genome correlated with a strain circulating in England. That suggested the patient had contact with someone who had brought the virus over from the U.K.

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The findings suggest that even after the Trump administration imposed travel restrictions from China, the virus continued to infiltrate the most populous U.S. city via daily flights from Europe.

Not all of the New York virus samples have European origins. Some appear to have come from the U.S. West Coast, while others appear to link directly to Asia. That indicates that there are numerous chains of transmission in the New York metropolitan area, as would be expected in such a large outbreak.

The NYU genome center previously had been focused on sequencing for common diseases such as cancer and heart conditions. But in early March it switched to working almost exclusively on COVID-19.

“We basically turned our labs into COVID-19 labs overnight,” said Heguy. “That is all we are doing right now.”

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