A new analysis of tests conducted at the UW Medicine Virology lab adds to the evidence that Washington is bending the curve on the novel coronavirus epidemic.

The percentage of specimens positive for the virus appears to have peaked on March 28 and has been trending downward ever since. If the trend continues, the rate of positive tests could drop below 2% by late April and — potentially — below 1% by mid-May. But reaching such low levels is dependent on maintaining the current level of social distancing.

“Our data indicate a true bending of the curve in prevalence of SARS-CoV-2 in the Seattle area and the state of Washington,” says the report from researchers at the University of Washington and the Fred Hutchinson Cancer Research Center.

On Wednesday, state health officials announced an additional 89 cases — the lowest number since March 7, when the epidemic was beginning to accelerate — and 26 deaths from COVID-19, the disease caused by the novel coronavirus. The new numbers bring Washington’s count of COVID-19 cases to 10,783, including 567 deaths.

The UW Virology lab has conducted about half of all coronavirus tests in Washington. It was one of the first nongovernmental facilities in the country to get approval to do the work, and as of April 14 had processed 66,027 specimens.

Samples are sent to the lab from clinics, hospitals and emergency rooms across Western Washington. While testing levels have been inconsistent and the UW data do not represent every corner of the state, they do provide one of the best indicators so far of the prevalence of infections, said co-author Larry Corey, a virologist and former director of The Hutch.


“I feel pretty confident it’s a reflection of what we call incident COVID cases, that we are seeing a true decrease in the COVID-positive diagnoses in our region and that it’s associated with … social distancing,” he said. “It’s a reflection that the people of the state are listening, and that the kind of measures put in place … seem to be working.”

The researchers analyzed more than 25,000 tests conducted between March 1 and April 8. They averaged three-day results to create smoothed curves and estimated that positive tests peaked between 12% and 13% on March 28. (Among emergency room patients, the highest single day rate was more than 35%; for outpatients, the highest single day rate was about 20%.)

On April 14, the UW lab reported 8.5% positives.

According to statewide testing data posted by the Washington Department of Health, the peak rate of positives was 14.5% on March 29. The number of new infections diagnosed is also dropping in much of the Puget Sound region. King County recorded a high of 207 new cases on April 1, with the number dropping to 65 by April 13. In Snohomish County, the number of new cases has fallen from a high of 137 on March 27 to between 51 and 32 in recent days.

While Corey said the test results reported in the new analysis are solid, he cautioned that projections showing the rate of positive tests dropping to 1% or 2% in the coming weeks have wide uncertainty ranges and assume the existing trend continues.

“It’s a prediction,” he said. “This is what it could look like, but it’s not reality.”

Even if the rate of positive tests dips below 1% by mid-May, that won’t mean the virus is gone, Corey said. With the majority of the people still lacking in immunity, infections could quickly begin to climb again. “We could have a rebound epidemic,” he said.

The report was submitted to the Journal of the American Medical Association but has not yet been peer-reviewed.

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