Detailed genetic analyses of the strains of virus that cause COVID-19 suggest that the outbreak took hold in Washington state in late January or early February, but went undetected for weeks.
That’s the upshot of two studies published by the journal Science, based on separate efforts to trace the genetic fingerprints of the coronavirus known as SARS-CoV-2.
The studies draw upon analyses of more than 10,000 samples collected in the Puget Sound region as part of the Seattle Flu Study during the early weeks of the outbreak, plus thousands more samples from other areas of the world.
One of the studies was conducted by a team including Trevor Bedford, a biologist at Seattle’s Fred Hutchinson Cancer Research Center who has been issuing assessments of the virus and its spread since the earliest days of the outbreak. The first version of the team’s paper went online back in March and was revised in May, months in advance of today’s peer-reviewed publication.
The other study comes from researchers led by the University of Arizona’s Michael Worobey, who also published a preliminary version of their results in May.
There are subtle differences between the two analyses. For example, Bedford and his colleagues say it’s most likely that the majority of the infections in Washington state arose from a single case that was introduced in late January or early February.