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An updated analysis of COVID-19 case data and Facebook mobility data suggests that Seattle and King County are “on the cusp” between wider spread of the epidemic and a gradual fade-out of new cases.

Researchers from the Bellevue, Wash.-based Institute for Disease Modeling said the decline in the rate of growth in new cases is a testament to the efficacy of Washington state’s “Stay Home, Stay Healthy” restrictions. But they also said it’s still too early to relax those restrictions.

“If distancing measures are relaxed without other mitigation strategies in place, we can expect a quick rebound in the case count and the burden on the hospitals, and the deaths,” Mike Famulare, principal research scientist at the institute, said today during a teleconference with journalists.

Public health officials agreed. Jeff Duchin, health officer for Public Health – Seattle & King County, said the infrastructure for conducting tests and tracing those who have been in contact with COVID-19 patients still has to be built out.

Previously: King County says social distancing is slowing coronavirus spread

“We should start thinking about the next steps in our transition away from the extreme social distancing measures,” Duchin said. “But it should be very clear that we are in no way ready to do that at this point.”

When asked to estimate the time frame for loosening restrictions, Duchin said that shouldn’t happen “within the next month, but it’s hard to predict beyond that.”

He also cautioned that life won’t return to normal, even when the shelter-at-home orders are lifted and businesses and schools reopen. People will have to be extremely careful about their interactions until an effective vaccine is widely available, which may not happen until a year to 18 months from now.

“It’s going to be a very slow and frustrating process for many, because these distancing measures are quite difficult. Unfortunately, the next year is probably going to be a very challenging year for all of us,” Duchin said.

The latest analysis estimates the effective reproduction number in King County as of March 25 at 0.73, with a 95% confidence interval ranging from 0.3 to 1.2. That key value, known as R e , describes how many new cases are spawned by each case that arises. If the number is greater than 1, that means the case load is rising. If the number is less than 1, that means the case load is diminishing.

Since March 25, social distancing measures may well have had an additional effect, but there’s not yet enough evidence to reach a conclusion. Based on data collected between March 25 and April 7, it looks as if mobility has settled into a stable pattern, with regular increases in movement during weekends.

As a result, the best guess is that the value of R e is fluctuating above and below 1. And for that reason, the researchers counsel caution. “While current levels of adherence to physical distancing policies are effectively controlling the rate of spread of COVID-19, the region is in a precarious state and must maintain distancing to prevent rebound transmission,” they write in their April 10 update.

Although the study focuses on King County, the research team says the course of the outbreak seems similar in Snohomish County to the north. To the south, Pierce County is following a similar pattern, but with a roughly one-week delay.

“In all three counties, we cannot say with certainty that R e is above or below 1,” the researchers say.

The top public health officials for Snohomish and Pierce counties said they agreed with the need for caution.

“There’s still a long way to go,” said Chris Spitters, health officer for the Snohomish Health District. “We’re not at the end of this by any stretch of the imagination. And I agree that we’re not even at a point where we can think comfortably about relaxing the social distancing measures.”

Anthony L-T Chen, director of health for Tacoma-Pierce County, saw good news in the fact that the number of news cases is no longer increasing exponentially. “The sobering news is that we are still increasing at a linear rate,” he added. “We may be getting closer to a peak, but we will not know for sure until we see a clear flattening and a downward trend in new cases for several days.”

Update for 4 p.m. PT April 14: We’ve updated this report with comments from this afternoon’s news briefing.

Authors of the Institute for Disease Modeling’s updated analysis, titled “Physical Distancing Is Working and Still Needed to Prevent COVID-19 Resurgence in King, Snohomish and Pierce Counties,” are Niket Thakkar, Roy Burstein, Daniel Klein, Jen Schripsema and Mike Famulare. Hat tip to The New York Times’ Mike Baker.