After dropping throughout March and into early April, the transmission rate of COVID-19 is no longer falling and could be rising again in western Washington, according to the latest report from Bellevue-based Institute for Disease Modeling (IDM).

A previous report issued last week found that the effective reproductive number—the number of new cases stemming from each COVID-19 infection—had dropped below the critical threshold of 1.0 in King County between March 29 and April 15.

However, updated case and mortality data from the Washington Disease Reporting System revised that estimate upward, showing the reproductive number had no longer been falling and likely has been inching up again since roughly April 6.

Here’s more from Public Health – Seattle & King County:

As of April 22, the number of new cases from each COVID-19 infection in King County was between 0.47 and 1.32 (best estimate 0.89). Overall in western Washington, the reproductive number on April 22 was between 0.61 and 1.39 (best estimate 1.0).

No longer definitely below 1.0, cases in western Washington can be expected to plateau or increase, if the trend continues.

Physical distancing remains the best tool for reducing COVID-19 transmission. Because most of the population remains susceptible across the state, relaxing distancing policies will likely result in increased transmission. However, researchers’ ability to measure these increases are retrospective, delayed by weeks because of the time until symptom onset and delays in case reporting.

This report once again reminds us that our position is precarious and COVID-19 transmission and new cases remain unacceptably high,” said Dr. Jeff Duchin, Health Officer for Public Health – Seattle & King County. “We need to double down on distancing and other prevention steps at home, in the community, and in workplaces and we must see these numbers improve before relaxing our current restrictions.”

It’s clear that the course of the epidemic in Washington remains sensitive to changes in social distancing,” said Dr. Mike Famulare, Principal Research Scientist at IDM. “We all—scientists, policymakers, everyone in Washington—face a difficult challenge in the coming weeks as our day-to-day lives will be in flux as we are forced to respond quickly to slow transmission and save lives.”