New modeling suggests transmission of the novel coronavirus has slowed more than expected in King, Pierce and Snohomish counties due to social distancing, and may have reached a critical threshold.

But the progress remains precarious, with evidence that even increased movement of people on the weekends is enough to nudge the rate back up.

“The new report shows that … we are getting very close to that landmark where each person who’s infected spreads the disease to one or fewer others, and that is what is really needed to stop the transmission of the virus,” Dr. Jeff Duchin, health officer for Public Health – Seattle & King County, said Tuesday in a briefing with his counterparts from the two neighboring counties.

Earlier in the epidemic, the so-called effective reproductive number was nearly 3, meaning each infected person was passing the virus on to almost three other people. The rate of new infections was rising exponentially.

Now, the number of new cases in King County has remained steady for nearly two weeks, while in Snohomish County the new infection curve is bending downward. But in order to bring the epidemic under control, the effective reproductive number must remain below one.

Researchers at the Institute for Disease Modeling (IDM) in Bellevue estimate it may have dropped to around 0.75 across the three counties, but the uncertainty is high, with a possible range of 0.5 to one.

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“We’re not … at the point where we can think comfortably about relaxing social distancing measures,” said Dr. Chris Spitters, health officer for the Snohomish Health District. “But if things continue in this direction, I think that is on the horizon and how we go about doing that is very important.”

Duchin said it’s likely existing restrictions will need to remain in place at least another month. And each time social distancing is eased, in a stepwise fashion, it will take several weeks to evaluate the impact before loosening things up further.

“The virus isn’t going anywhere, and the vast majority of us are still susceptible,” he said.

A group of epidemiologists from Harvard University’s T.H. Chan School of Public Health reported Tuesday that their models suggest off-and-on intervals of social distancing may be required into 2022 to prevent hospitals and intensive care units from being overwhelmed by COVID-19 patients.

Looking ahead in Washington state, the best way to decide which activities and business to open first is to look at measures that can be applied to ensure the safety of workers and the public, said Dr. Anthony L-T Chen, director of health for the Tacoma-Pierce County Health Department.

For example, restaurants might be allowed to resume dine-in service, but with fewer tables to maintain social distancing. Barber shops might leave half their chairs empty and ask customers to wait in their cars.

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And it’s important to consider the impact on lower-wage employees who are already “about to fall off the cliff if they don’t get some relief quickly,” Chen said.

As social distancing is eased, control of the virus will depend on a more targeted approach. Newly infected people will have to be identified quickly and isolated, along with their contacts, to prevent continued flare ups. So it’s important not to lift restrictions until the number of new cases is at a manageable level, Spitters said.

In Snohomish County, that would be far below the current rate of about 30 new infections a day.

Testing capacity also needs to expand at the same time local health departments add staff to allow them to do the work of contact tracing.

Investigating just a single case, which includes tracking down everyone who might have crossed paths with the infected person, can take an entire day, Spitters said. By one estimate, Washington will need hundreds of additional health workers to tackle the job, Duchin added. Massachusetts, with a population similar to Washington’s, announced plans to deploy 1,000 disease trackers.

An alternative approach championed by tech companies like Apple and Google would use digital tracking based on cellphone apps. Duchin said he and his team have been advising developers on ways to make their products useful to public health departments, but it’s not clear they will succeed.

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Chen said it makes sense to rely on more low-tech approaches, such as call centers, where people with limited training can do much of the initial contract tracing, leaving the complicated cases to the professionals. Pierce County has already mobilized Medical Reserve Corps volunteers – people with and without health-care experience – to help with the public-health detective work.

The new modeling results released Tuesday build on previous work by IDM that estimated social distancing had roughly halved the transmission rate by mid-March compared to 2.7 new cases per infected person in late February. As before, the group used anonymized location data from Facebook users to track changes in mobility as a proxy for social distancing and extended their calculations to March 25.

While movements plunged steeply beginning in late February, the researchers found there wasn’t much change after Gov. Jay Inslee imposed a “stay-home” order on March 23. Nevertheless, transmission of the virus apparently continued to decline, possibly reflecting more conscientious application of measures like hand-washing and mask-wearing.

Looking forward, projections show that the effective reproductive number is likely to fluctuate, rising on weekends when more people are moving around. And if social distancing is relaxed now, said IDM’s Mike Famulare, there’s little doubt the disease will rebound quickly.

The Harvard team, whose analysis was published in the journal Science, ran computer simulations of a range of scenarios and found one of the most important variables, as yet unknown, is how long people remain immune after being infected. They also found that development of treatments and a vaccine could significantly reduce the amount and intensity of social distancing required, by reducing the number of people sick enough to need hospitalization.

However, they found that even if the virus has a seasonal pattern like influenza, infections will continue through the summer months because so many people remain vulnerable. Another peak is almost certain to follow in the fall and could be even worse than the initial wave, particularly if onetime social-distancing measures are lifted abruptly.

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The modelers stressed that the course of the disease will play out differently in different locations. “These epidemics are really taking place locally and are subject to the types of interventions that happen locally,” infectious disease specialist Yonatan Grad said in a media briefing.

For Western Washington, that’s likely to mean a long, uncomfortable stretch as normality returns in halting steps, Duchin said.

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

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