I've been following these guys for a little while. A team in Seattle has been doing retrospective testing of samples and developing a phylogenetic tree to help determine when the virus likely reached the United States.
The conclusion is upfront in the below twitter thread. (18 tweets)
A key point: They tested 3600 samples taken in January and 3308 in February as part of an existing, ongoing flu/respiratory project in Seattle. They found zero positives in January, and their first positive on February 21. The non-Covid samples displayed a wide range of other respiratory illnesses.
A second key point. They looked at hundreds of sequenced viruses from Covid infections in the US. They tracked sequences to China, with an introduction in the US in late January. They tracked multiple introductions from Europe in February.
The conclusion is upfront in the below twitter thread. (18 tweets)
A key point: They tested 3600 samples taken in January and 3308 in February as part of an existing, ongoing flu/respiratory project in Seattle. They found zero positives in January, and their first positive on February 21. The non-Covid samples displayed a wide range of other respiratory illnesses.
A second key point. They looked at hundreds of sequenced viruses from Covid infections in the US. They tracked sequences to China, with an introduction in the US in late January. They tracked multiple introductions from Europe in February.