Time for this week’s Washington update. I’m tracking the daily positivity rate for Washington State using data from the Washington Department of Health, the University of Washington Virology Laboratory and the Snohomish Health District.

The daily positivity rate is the percentage of positive COVID-19 test results for each 24 hour period. The rate adjusts for the number of tests which varies from day to day.

The daily positivity rate is a useful metric, but an imperfect one. As I’ve said in previous posts, we need a proper epidemiologic study of COVID-19.

The first graph (below) is the daily positivity rate for Washington State though April 15. Why April 15 and not today, 24 April 2020? The Department of Health does not guarantee complete data for the most recent days. The data through April 15 is (nearly) complete although a few test results still trickle in.

As noted in the graph posted last week, the state is past the peak and is in a slow decline. The peak occurred in late March. The daily positivity rate has declined to where it was in mid-March.

For comparison, here is the data from the UW Virology Lab. UW performs testing for and county and state health authorities. [Click images to enlarge.]

The shape of the graph is consistent with the overall state data. The Virology Lab data is up-to-date since they just report daily results for the lab itself (samples in, results out).

The Washington State epidemiologic curve (below) is informative, too. Known cases are tallied by the date of illness onset.

The epidemiologic curve shows a steady decline due to community mitigation, also known as “social distancing.” This is good news.

To complete the picture, here is the epidemiologic curve for Snohomish County, where I live.

Snohomish County shows a similar decline in new confirmed cases. Again, this is good news.

The big question for Governor Inslee and other decision makers is “How low is enough?” The answer to this question determines when social distancing can be relaxed. Currently, social distancing is expected to be in place until May 4th at the earliest. Hopefully, Washington will beef up contact tracing and isolation over the next two weeks. Then, it’s whack-a-mole.

Stay distant and stay healthy. Science works — P.J. Drongowski