WHEN A group of public health experts sat down last year to imagine a pandemic caused by a highly transmissable respiratory virus, they foresaw much of what has occurred in the past few months. The experts, writing for the Johns Hopkins Center for Health Security, predicted health-care systems could be overwhelmed, medical supply chains stressed and the need for vaccines and drugs urgent. But in one area, they were uncertain — would social distancing work?

The experts, in a report published last September, predicted that governments would turn to social distancing and other so-called non-pharmaceutical interventions, or NPIs, including limits on travel and strict quarantines. But they said not enough was known about whether such measures would be effective, and expressed concern about downsides. While governments would naturally act to reduce contact among people to slow the disease spread, they wrote, “there is a broad lack of evidence of efficacy and a lack of understanding about secondary adverse impacts.” They were particularly critical of quarantines such as that attempted in West Africa during the Ebola outbreak, which turned chaotic. Their views also reflected a long-standing assumption that authoritarian regimes could more readily implement strict social distancing, while it would be nearly impossible for democracies.

One of the most surprising lessons of the coronavirus pandemic so far has challenged this assumption. Social distancing has never been tried on such a scale as now, and there is evidence that it can work, in democracies and dictatorships. Where people have followed the guidance to stay home, close schools, refrain from going into restaurants and bars, shutter workplaces and businesses, they have begun to slow the growth in infections, hopefully averting hospital overload. Gov. Gavin Newsom (D) of California, who imposed a statewide stay-at-home order, said Monday, “We know what does work, and that’s physical distancing.” In Seattle, officials said strict containment strategies imposed in the earliest days of the outbreak were paying off. Hospitalizations are starting to decline. A company that charts reports of feverish illness levels across the country — fever is an early symptom of coronavirus infection — found that once social distancing began, the level of observed illness in the population declined.

No one can declare victory. The strategy of “flattening the curve” doesn’t work if people ignore the need to practice social distancing and thus allow the virus to spread. If politicians and people ignore the restrictions, or tire of them too soon, more cities will flare like New York. A research paper by Michael Greenstone and Vishan Nigam at the University of Chicago, based on previous modeling of the pandemic, concluded that moderate social distancing “would save 1.7 million lives between March 1 and October 1, with 630,000 due to avoided overwhelming of hospital intensive care units.” Even if social distancing achieves only a fraction of that, it seems to be worth a try.

Yes, the distancing is disruptive and demands sacrifice. The only worse outcome is a failure to distance, more contagion and even higher costs in lives, treasure and prolonged crisis.

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