Social distancing is controlling COVID-19; now scientists need to figure out which measures are most effective

A person goes for a walk in Brooklyn last weekend with the New York City skyline in the background. Spencer Platt / Getty Images

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Banning gatherings, having people stay at home, closing schools, and other measures to reduce the spread of Covid-19 are working so well that an influential model of the pandemic’s course now projects that the number of U.S deaths from the new coronavirus by early August will be 60,415 rather than the minimum of 100,000 that it forecast last month.


The new, lower estimate is prompting officials to be cautiously optimistic about controlling the outbreak, possibly by early summer. But it comes with a mystery: Which social distancing has reduced transmission the most?

Figuring that out will be crucial as policymakers look for ways to lock in the progress against Covid-19 while eventually re-opening schools and the economy, an imperative that has become even greater in light of the more than 17 million people who have lost their jobs due to the epidemic.

The earlier projection of Covid-19 deaths — 100,000 — from modelers at the Institute for Health Metrics and Evaluation at the University of Washington assumed there would be broad adoption of “social distancing,” which is also how China broke the back of the epidemic that began in Wuhan. But the effect of that social distancing has been greater than the IHME team expected.

“This should be entirely obvious,” UW epidemiologist Trevor Bedford tweeted on Wednesday night. “But we now have strong evidence that social distancing results in decreased Covid-19 transmission rates.”

In epidemiologists’ terms, the number of cases caused by each infected person, called the effective reproduction number (Re), has fallen precipitously. If people’s contacts drop by a certain amount, then so does the the reproduction number, said epidemiologist Gerardo Chowell of Georgia State University. The new IHME projection, he said, “looks reasonable.” As additional data become available, the model “can provide a better view of the trajectory of the epidemic in the near future.”


In the U.S., Re has fallen most clearly in Seattle’s King County. There, researchers at the Institute for Disease Modeling calculated, it fell from 2.7 (and possibly as high as 3.5) in late February, when community transmission was accelerating, to roughly 1.4 or even lower on March 18. The county would have had roughly triple the number of cases in late March without social distancing, they found.

In 11 European countries, modelers at Imperial College London estimated in late March, Re has fallen from 3.87 to 1.43, averting 59,000 deaths, and possibly 120,000, just through the end of March.

The new Re in Europe as well as King County, and probably elsewhere, is encouragingly close to 1, the level at which the outbreak will burn itself out.

Like other countries, the U.S. has taken an everything-but-the-kitchen-sink approach to social distancing: close schools and ban many international arrivals and close restaurants and ban church services (and other events that bring people together, even funerals) and order those who can to work from home. As a result, officials can’t tell which measures have been the most effective.

It is also difficult to “decouple” the effect of those government policies from any voluntary measures people took to protect themselves, health economist Rahi Abouk of William Paterson University and computational social scientist Babak Heydari of Northeastern University argue in a paper posted to a social science site on Thursday.


The reason it is so important to determine which interventions lower people’s contact rate the most, Heydari said, is that “policies in many states are still evolving.” In those that have laxer policies but may be about to experience a surge in cases, he said, that understanding “can help them update their policies. [And] it is likely that some states, or the country as a whole, will experience subsequent outbreak waves, after they phase out some of the [current] policies. These results can inform reinstating” social distancing measures.

For instance, projections released this week by researchers at Columbia University identify U.S. counties at risk of seeing their health care systems overwhelmed by Covid-19 cases. The researchers ran models in which they assumed that people’s contacts dropped by 20%, 30%, or 40%. The latter could avert as many as 185,000 deaths in the Northeast and 33,000 deaths in the Midwest. But the model is agnostic on what mix of business closings, work-at-home policies, and other measures achieve any given level of social distancing.

So is the IHME model. That makes it difficult for policymakers to tell whether relaxing one form of social distancing while tightening others might bring the same benefits, at perhaps a lower economic cost, or to know which forms of social distancing can be eased first in an effort to tiptoe toward normalcy, Chowell said.

Results from preliminary studies are starting to provide some answers.

In research posted to the preprint site medRxiv this week, Chowell and his colleagues show that “the pandemic may be readily controllable through a combination of testing, treatment if necessary, and self-isolation after testing positive of symptomatic individuals together with social protection” such as face-mask use and hand-washing.


When the basic reproduction number is as high as 2.4, which King County and some areas in the U.S. are likely well below, 65% effective social distancing (that is, people have only 35% of their previous number of close contacts) brings Re below 1, the magic “this is ending” number. But social distancing that’s only 20% effective (people have 80% of their usual close contacts) also brings Re below 1 if 75% of infected and symptomatic people are identified and isolated within 12 hours of symptom onset.

“Sustained universal mask-wearing and frequent hand-washing have the potential to dramatically reduce the transmission rate of the virus after the first wave,” Chowell said. They “have the potential to help us safely return to work and school,” though he’s in favor of “remote learning and telework as much as possible.”


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