Although many states are starting to relax restrictions on business activity and contacts, Seattle-area epidemiologists say such restrictions will need to be tightened up to reduce the spread of coronavirus much further.

The only alternative to clamping down harder would be to create a robust system of testing and contact tracing, experts at the Bellevue, Wash.-based Institute for Disease Modeling say in their latest report.

Their conclusions are based on an updated analysis of viral transmission patterns in Seattle and the rest of King County. The numbers suggest that the extent of the pandemic in the county is declining very slowly, if at all.

In technical terms, the effective reproductive number for the virus’ spread, also known as Re, has declined from 3 in early March to somewhere around 1 as of April 4. The best estimate is 0.94, with an uncertainty interval of 0.55 to 1.33 to a 95% confidence level.

Re measures how many new infections arise from each existing case. When Re equals 1, that means every person with the virus would on average transmit the virus to one new person, resulting in a stable population of infected people.

Today’s tally reports 5,990 confirmed COVID-19 cases in King County, including 416 deaths. Over the past couple of weeks, the day-by-day figures show a gradual decline, punctuated by midweek spikes that are probably due to reporting patterns.

The institute’s epidemiologists warn that if restrictions are loosened starting on May 1, as a number of states are planning to do, their computer model predicts that new COVID-19 cases would reach a new peak by as early as the end of May. But if new strategies are added, the number of new cases should drop quickly.

Jeff Duchin, health officer for Public Health – Seattle & King County, said the Seattle area is still at a turning point in the course of the pandemic.

“Although we have significantly decreased the spread of COVID-19 through stay-at-home and distancing, the daily number of new cases remains unacceptably high, and our community remains vulnerable to a rebound in cases that could overwhelm our health care system if we change course too quickly,” Duchin said in a news release. “We are working hard to better understand how and where new infections are happening, which may suggest other measures that can help reduce transmission.”

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Duchin said there’ll be a continuing need for at least some physical distancing until effective treatments and vaccines for COVID-19 are available — which could take a year or longer. “Increased testing and contact tracing will be critical for us to manage this illness in our community going forward, and which should also further reduce transmission,” he said.

Mike Famulare, principal research scientist at IDM, seconded Duchin’s view.

“The actions taken in King County have prevented much suffering from COVID-19, but transmission still persists,” Famulare said in a statement. “To prevent a resurgence of infection and preventable deaths, social distancing remains necessary to keep the transmission rate low until additional measures, including testing or contact tracing, can be significantly scaled up.”

In their previous reports, Famulare and his colleagues factored in Facebook mobility data to predict the course of the coronavirus outbreak. But in the latest report, issued Friday, the epidemiologists say those mobility statistics have lost their predictive power because they serve only as a coarse measure for the impact of physical distancing. IDM researchers are currently looking for data sources that measure the effect of stay-at-home orders more precisely.

Over the past few days, Washington Gov. Jay Inslee has announced measures to loosen restrictions on business and leisure activities, ranging from construction projects to outdoor recreation.

We’ve reached out to the governor’s office to find out how those policy changes square with the IDM recommendations — and we’ll pass along anything we hear.

In addition to Famulare, the authors of IDM’s latest analysis of COVID-19 transmission in King County include Niket Thakkar, Roy Burstein, Jen Schripsema and Daniel Klein.