[jlmorton]

> This sounds like circular logic - how we know it indeed started in January and not before?

Indeed, without serological testing, we're necessarily making some assumptions. But there's other evidence that points this way, too.

Genetic analysis based on samples from around China and the world point to a single index patient in Hubei Province around late November. China's case tracing has not gotten to an index patient, but it appears to be pretty close, and also points to an initial case around the same time.

If there was a single case in late November, then we'd expect a few hundred cases by January. Some number of these will have traveled, but many would not.

While the US clearly had major blind spots in testing, we were explicitly looking for symptomatic cases from China. The first detected case was Jan 21. There may have been a few earlier, but genetic analysis of cases in Washington point to an index patient in the Seattle area around that time.