Blood sample analysis shows that coronavirus arrived in the US in December 2019

A new analysis of blood samples from 24,000 Americans taken in early 2020 is the latest and largest study to suggest the coronavirus arrived in the US in December 2019.

That's weeks before cases were first recognized by health officials. The analysis is not definitive, and some experts remain sceptical. But federal health officials are increasingly accepting a timeline in which small numbers of Covid-19 infections may have occurred in the US before the world became aware of a dangerous new virus erupting in China.

The study was published on Tuesday online by the journal Clinical Infectious Diseases.

“The studies are pretty consistent,” said Natalie Thornburg of the Centers for Disease Control and Prevention. “There was probably very rare and sporadic cases here earlier than we were aware of. But it was not widespread and didn't become widespread until late February.”

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A CDC-led study published in December 2020 analyzed 7,000 samples from American Red Cross blood donations and suggested the virus-infected some Americans as early as the middle of December 2019.

The latest study is by a team that includes researchers at the National Institutes of Health. They analyzed blood samples from more than 24,000 people across the country, collected in the first three months of 2020, as part of a long-term study.

The coronavirus emerged in late 2019 in Wuhan, China. Officially, the first US infection identified was a traveller — a Washington state man who returned from Wuhan on Jan. 15 and sought medical help on Jan. 19.