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Seattle area COVID-19 tests show infections declining

Trends in coronavirus test results suggest social distancing is working
Working in isolation.
Working in isolation. Whether laboring from home or in offices, social distancing is the new norm in Seattle, and a new study shows it may be helping to drive down COVID-19 infection rates. Photos by Robert Hood / Fred Hutch News Service

For all those living in the Seattle region hunkering down in their homes, there is new evidence that social distancing is reversing coronavirus infection rates, a green shoot of hope in this frightening spring of 2020.

After analyzing results of more than 25,000 nasal swab tests taken in hospital emergency rooms and daytime clinics in the greater Seattle area, researchers have found compelling evidence that COVID-19 infections in the heart of Washington state’s outbreak peaked roughly on March 28 and are now in steady decline.

At that apparent apex of infections, about 13% of those tests taken outside of hospitals were coming back positive. If that downward trend — and rigorous physical distancing — continues, researchers predict that by mid-May as few as 2% of tests in the region will be positive.

“This is the first objective data that suggests the incidence of infection appears to be decreasing here,” said Dr. Larry Corey, past president and director of Fred Hutchinson Cancer Research Center, a professor in its Vaccine and Infectious Disease Division and an author of the unpublished study.

“It’s gratifying to see that the sacrifices people are making are having results,” he said. “I want to thank the people of this region, the governor and the mayor for how they have responded to this crisis.”

graph showing decline in positive test results
A graph tracking the percentage of daily COVID-19 tests that were positive shows the rate rose rapidly in early March. It peaked at the end of the month at about 13%, and it has been declining ever since. The model projects the rate could go below 2% by mid-May. The pink shadowing indicates the degree of uncertainty, which increases noticeably when extrapolated beyond the study period ending April 8. Courtesy Vaccine and Infectious Disease Division, Fred Hutch

Corey said the researchers have submitted their findings in a letter to an academic journal, but felt the data are of major interest to the general public and should be shared as quickly as possible.

The work is a collaboration among teams led by Dr. Keith Jerome, a professor at both Fred Hutch and the University of Washington who directs the UW Virology Lab; and Dr. Alex Greninger, assistant professor at UW’s Department of Laboratory Medicine. The analysis was done by a Fred Hutch statistical team led by Drs. April Randhawa, Leigh Fisher, Shuying Li and Jessica Andriesen.

The researchers collected data from tests involving 25,486 individuals, mostly in King and Snohomish counties who were treated as outpatients — those visiting various clinics but not hospitalized. A smaller contingent tested had visited emergency rooms in the same region. Thus, the researchers had data on patient demographics and site-of-origin data for the samples.

As a way to track the epidemic, Hutch-based statisticians evaluated the rate of positive COVID-19 patients coming to these clinics each day. The study began on March 2, the first day that regional SARS-CoV-2 detection was initiated in the UW Virology Lab, and it tracked the rate of positivity until April 8.

“We reasoned that the analysis of the rate of COVID positivity in outpatient and ER facilities with established connections to the laboratory would allow estimates of COVID-19 incidence in our region,” Corey said.

Infection rates could decline significantly in coming weeks

Initially, about 4% of tests turned up positive. But the regional epidemic sharply increased, peaking the weekend of March 28-29, when nearly 13% of samples submitted were positive for COVID-19. Those numbers have since fallen to about 7%, providing supporting evidence that physical distancing seems to be having the intended effect.

These results come just after new modeling by the Institute for Disease Modeling suggested that the rate of transmission of COVID-19 has slowed more than expected in the same region.

Looking ahead, Corey said the mathematical model used to track the numbers so far suggest the decreasing rate of positive tests will continue, and using a conservative projection, could reach 2% by mid-May.

“It’s gratifying to see that the sacrifices people are making are having results. I want to thank the people of this region, the governor and the mayor for how they have responded to this crisis.”

— Dr. Larry Corey

A key assumption in the forecast of lower rates of infection is that the population in the Seattle area will keep up its disciplined social distancing well into May. “If you want to keep that nice, downward slope, we don’t start to open up,” said Hutch data scientist Andriesen.

Unlike the models used to predict caseload and deaths — those all-important curves that Americans are striving to flatten — the latest study focuses on those who feel sick enough to seek a test yet are not yet hospitalized.

“We’re testing the people in the middle,” Andriesen said.

That encouraging decrease in positive tests does beg the question: what is making people sick enough that they seek testing, even though an overwhelming majority of them find out, fortunately, that they do not have COVID-19?

“It means there are a lot of things out there with symptoms that overlap with COVID-19,” she said. “Was it flu, some other virus? The answer is, we don’t know yet.”

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Last Modified, April 16, 2020