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The US missed a chance to catch imported cases of coronavirus earlier this year, genetics experts say in a new report.

Analysis of the virus by evolutionary biologist Michael Worobey of the University of Arizona and colleagues shows the first person known to have carried the infection to the United States, in Washington state back in January, may not have been the source of the later cases there.

“Our analyses reveal an extended period of missed opportunity when intensive testing and contact tracing could have prevented SARS-CoV-2 from becoming established in the US and Europe,” they wrote in a report, which is not yet peer-reviewed and published on the preprint server bioRxiv.

Worobey and colleagues compared the strains of the virus that infected the first known US case, a 30-year-old traveler from Wuhan who arrived on January 15, and samples taken from a cluster of cases that popped up in the state at the end of February.

Genetic sequencing showed the samples were very similar, a finding that led officials to believe the first patient must somehow have spread the virus. 

But Worobey said scientists have since found that coronavirus mutates very slowly. He and colleagues ran computer simulations that showed there was no way the virus that came into the country with the first patient in January was the same virus infecting people six weeks later, even though it was very similar.

They concluded there had to be a second introduction of the virus in mid-February. “It looks like early to mid-February appears to be the time frame when this virus was introduced to Washington state,” Worobey told CNN.

“It seems that a virus that was probably identical to the Washington epidemic cluster got in at some point in early February and gave rise to a bunch of identical viruses,” said Joel Wertheim of the University of California San Diego, who also worked on the study.

Worobey said the second introduction of the virus may have come from Canada.

“It seems possible that the virus that eventually wound up in Washington state moved first into Vancouver and then down into Washington,” he said. “We can’t exactly say when. We can’t say who. We can’t say where from.”