After making slow progress against COVID-19, Washington State lost a little ground during the last two weeks. We aren’t heading up the steep slope, but the unusually good weather has brought people out of isolation.

First up is the graph of the daily positivity rate using data from the Washington State Department of Health. [Click images to enlarge.]

We can see a leveling off in the positivity rate during in the last week of April 2020.

For recent preliminary results, I turn to data from the University of Washington Virology Laboratory (UW). UW performs testing for the state — on the order of 25% of the tests performed each day.

Here we see a more definite up-tick in recent days. As a state, we need to double-down on social distancing — it’s too soon to give up! I don’t think anyone really wants to go through another long lock-down. It’s better to quash COVID-19 and keep it down. Anything less delays economic recovery.

The Institute for Disease Modeling (IDM) estimates the effective reproductive number (R_e) for King County, Washington. King County is the most populous county in Washington covering much of the Seattle metropolitan area. (Snohomish County fringes on the metro area, it’s main population center is Everett.)

IDM reports both a likely range for R_e (95% confidence interval) and a best estimate for its value on a particular date. The table below summarizes their findings:

    Date            Lower  Upper  Best Est
    --------------  -----  -----  -------- 
    March 25, 2020   0.30   1.20      0.73 
    April 4, 2020    0.55   1.33      0.94 
    April 15, 2020   0.28   1.00      0.64 
    April 27, 2020   0.47   1.32      0.89 

IDM base their analysis on data from the Washington Department of Health and mobility (traffic) data. IDM’s best estimate of cumulative incidence through April 20 is 2.1%, meaning that 2.1% of the community has or has had COVID-19 as of April 20.

Worrisome, IDM hasn’t found R_e to have dropped definitively below one for Eastern Washington. Although Western Washington has made progress, Eastern Washington is not improving. The number of new cases tracks Re. When R_e increases, the number of new cases increases, too, lagging in time by the COVID-19 incubation period.

Please remember that we are shooting for an R_e less than one, meaning that the spread of SARS-CoV-2 (the virus which causes COVID-19) is slow enough to prevent a rapid rise in new cases. Social distancing, contact tracing and isolation are all about keeping R_e as far below one as possible.

Keep the faith — P.J. Drongowski