Results of first-ever home testing for COVID-19 in Seattle and King County indicate infections are declining but that there still may be thousands of residents infected with the SARS-CoV-2 virus undetected in the community, according to a report released Friday by The Greater Seattle Coronavirus Assessment Network (SCAN), which has analyzed the first round of “swab and send” testing kits from 4,092 county residents.

The early findings come from the first 18 days (March 23-April 9) of data collected by SCAN, a Bill Gates-funded partnership between the scientists behind the Seattle Flu Study and Public Health – Seattle and King County, being implemented by the Brotman Baty Institute. The research involved sending newly developed self-test kits to individuals who registered with the group online, then analyzing the results. It is the nation’s first home-surveillance project on COVID-19 and, while many of the early findings are preliminary and the margins of error wide, the report says the data provides the first real snapshot of the undetected spread of the virus through the community and will be refined as testing expands.

The goal of the study is to give health officials a clearer understanding of how far the virus has penetrated the community and answer the question: “Is there an iceberg of cases below the currently recognized tip?”

Of the 4,092 King County residents who registered for SCAN during that period, fully 2,700 of them had reported having COVID-like illness (fever, cough or shortness of breath) in the past seven days. Of those, 44 tested positive for the disease, according to the report. 

Officials note that incidence of infection is lower than is being seen within the medical system, where more seriously ill people wind up, but “may still represent thousands of unrecognized infections in the community,” according to a Public Health news release. The county had identified 4,902 cases as of Friday, with 330 deaths.

Overall, SCAN’s data suggests that somewhere between 5 and 75 of every 10,000 King County residents are infected, with the best estimate being roughly 24 per 10,000 residents, or 0.24%. The report said cases were identified throughout the county.


“In other words, at any given time during that period, we estimate the infected population in King County was between 1,100 and 16,900 individuals,” the report concluded. Among people who reported COVID-like symptoms, the prevalence of infection was 1.47%, the report said.

However, SCAN said there is evidence that those rates may have been declining — if so, the decline would almost certainly be a result of social distancing and the closure of most businesses and public places.

The study divided the 18 days into three six-day increments and the data shows a sharp decline in the prevalence in the community from one period to the next, dropping from around 0.32%  from March 23 through March 28; to around 0.27% from March 29 through April 3; to 0.07% for the six days ending April 9. At that 0.07% rate, the report estimated the number of COVID-19 cases in King County would not exceed 8,900.

“Although the trend is decreasing, the team cautions that the conclusion is uncertain and not statistically significant given the sample size,” the county health agency said in its news release.

Jay Shendure, scientific director of the Brotman Baty Institute and SCAN’s lead partner, said the team is encouraged by the community’s interest in SCAN and is working to get more tests in the hands of residents in coming weeks, particularly younger people and children, who in some cases have few or no symptoms but are capable of spreading the highly infectious novel coronavirus.

“The extent of positivity in kids, including within different age ranges, may inform our understanding of the relative impact of school closures vs. other social-distancing strategies, as well as the risk of rebound when schools reopen,” the report states.


In the long term, the self-test SCAN data will be used to track levels of infection as social distancing and other mitigation efforts are relaxed. Since it offers testing to people who don’t feel sick enough to seek health care, it also will help officials track otherwise undetected transmissions, the report says.

SCAN is the progeny of the Seattle Flu Study, which began in 2018 and pioneered a method of using volunteers and self-administered tests to track community spread of influenza and respiratory viruses. When the new coronavirus flared, flu study researchers quickly developed a test for the pathogen and began finding positive cases among nasal swabs they’d already collected.

But the team ran into regulatory hurdles. Though it was certified for research testing, it was not certified as a clinical laboratory. The New York Times reported state regulators, enforcing federal rules, shut down the lab’s coronavirus testing March 9 — a time when federally sanctioned test kits were in short supply.

The SCAN project’s self-swab system has now been approved by the Washington State Department of Health for emergency COVID-19 testing, and the lab where the swabs will be processed has been federally certified, said Shendure, who is also lead principal investigator for the Seattle Flu Study.

Material from The Seattle Times archives is included in this report.


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