Helen Branswell writes:

New research has poured cold water on the theory that the Covid-19 outbreak in Washington state — the country’s first — was triggered by the very first confirmed case of the infection in the country. Instead, it suggests the person who ignited the first chain of sustained transmission in the United States probably returned to the country in mid-February, a month later.

The work adds to evidence that the United States missed opportunities to stop the SARS-CoV-2 virus from taking root in this country — and that those opportunities persisted for longer than has been recognized up until now.

“Our finding that the virus associated with the first known transmission network in the U.S. did not enter the country until mid-February is sobering, since it demonstrates that the window of opportunity to block sustained transmission of the virus stretched all the way until that point,” the authors wrote in the paper. The paper has been posted to a preprint server, meaning it has not yet been peer-reviewed or published in a journal.

The research was led by Michael Worobey, a professor of evolutionary biology at the University of Arizona.

Using available genetic sequence data, Worobey and his co-authors modeled how SARS-CoV-2 viruses would have evolved if the original case, known in the medical literature as WA1 (short for Washington state patient 1), had been the source of the state’s outbreak. They ran the model 1,000 times, comparing the genetic sequences of 300 randomly selected simulated cases to those retrieved from 300 actual patients. The results didn’t gibe.

“In all likelihood this didn’t start with WA1,” Worobey told STAT in an interview. “It started with some unidentified person who arrived in Washington state at some later point. And we don’t know from where.” [Continue reading…]