New York Mayor Bill de Blasio announced this week that he was forced to close public schools—serving 1.1 million children—after the city’s coronavirus “positivity case” rate hit a seven-day rolling average of 3%. At a Wednesday afternoon news conference, Mr. de Blasio cited a “data-driven, science-driven” decision-making process. The school system, which had opened for part-time, in-person classes only six weeks ago, shut its doors the next day.
The decision angered parents, as the virus hasn’t been spreading in schools. Tens of thousands of children and staff have been tested for Covid-19, with a reported case rate of only 0.19%. The school system is thus 15 times as safe as the city at large, so it makes little sense to close the schools to fight a rising second wave of infection. Most European countries have kept schools open even while businesses remain closed. The World Health Organization advises that there have been only limited cases of student-to-student transmission, and that school closures aren’t an effective means of reducing community transmission. The Centers for Disease Control and Prevention has similarly concluded that the minor risks of keeping schools open for in-person learning are outweighed by the social and economic costs of closing them.
In New York, however, both Mr. de Blasio and Gov. Andrew Cuomo insist that the 3% seven-day rolling average of positive cases has some concrete bearing on policy. “We set a very clear standard and we need to stick to that standard,” the mayor explained, even as reports emerged that his own health officials disagreed.
In fact, the 3% mark is arbitrary—a standard plucked from thin air. Case positivity is a useful measure for gauging whether viral spread is increasing or decreasing. As a stand-alone indicator, it means next to nothing. Much depends on how many people are being tested. Unlike hospital admissions or deaths, case-positivity measurements can be tweaked downward by expanding the number of people getting tested—the denominator in the ratio. Mr. de Blasio alluded to this when he established “Get Tested Tuesdays” and hectored New Yorkers to get tested and retested even if they had no symptoms.
But if case positivity is too weak a metric by which to make major decisions, why was it the sole standard used to close the schools? Asked about this on Wednesday, Mr. de Blasio offered a decidedly unscientific answer. “We set a standard of 3% to keep faith with our families and with our educators and our staff,” he said. “That was the promise I made to everyone . . . so I think it’s important to keep consistency with that promise as we’re dealing with this new challenge.”
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