First results from the SCAN Study of #COVID19 transmission in King County, Washington, have been released. Overview here: publichealthinsider.com/2020/04/17/gre… and technical report here: publichealthinsider.com/wp-content/upl…. 1/8
These samples were collected in an innovative fashion, where individuals can sign up online at scanpublichealth.org and get a self-swab kit sent to their door and then have it delivered back to the lab for testing. 2/8
I believe this "swab-and-send" strategy could be leveraged to scale up testing in other locations to avoid an individual having to present to clinical care in order to get a test done (in addition to other clever approaches like drive-through testing). 3/8
These results show a pretty dramatic decline in the proportion of swabs from symptomatic individuals that are positive going from 2-3% on March 23-28 to 0-1% on April 5-10. 4/8 
Converting sample proportion to population prevalence is challenging and requires multiple assumptions. Here, this estimate declines from ~0.3% to ~0.1% between March 23 and April 9. Between April 4 and 9, absolute prevalence is estimated at 1600 to 8100 active infections. 5/8
Although absolute numbers are difficult to have full confidence in, we can trust relative changes through time. The rapid decrease in population prevalence fits with modeling results for estimating Rt through time in Washington based on case data. 6/8
Results from an infectious disease surveillance platform like SCAN can provide situational awareness regarding disease spread as social distancing begins to relax, so that we're not caught unaware by spread within the community. 7/8
Fantastic work by the SCAN team, lead by @DocJeffD and @JShendure, and including Karen Cowgill, Stephanie Schrag, @royburstein, @debnick60, @mjonasrieder, @HelenChuMD, @lea_starita, @famulare_mike, @KiraNewmanMDPhD, @trs, @databae_ , @misja_ilcisin 8/8
(I recognize this result is seemingly at odds with seroprevalence result from Santa Clara. Trying to think through this now.)
Here are the thoughts on Santa Clara: . I suspect the resolution is somewhere in the middle. Besides different study populations, antibody may be picking up more false positives and PCR may be getting more false negatives.
