Where did NYC’s 3% school shutdown threshold come from, how well does it align with CDC guidelines, and should it be used to make citywide decisions?

By David Zweig

Depending on your Twitter feed and media diet, the phrase “3%, based on a seven-day rolling average” has become familiar. 

If this benchmark of COVID-19 positivity is met, as you likely know, New York City schools are supposed to close. 

Over the past couple of weeks, New York City has inched closer and closer. 

But where did this metric come from, and what science is it based on?

Back on July 31, Mayor Bill de Blasio announced that the schools would only be allowed to reopen if the citywide positive test rate was below 3%, based on a seven-day rolling average — and would shut down if the city reached the same threshold. 

The scientific basis for this metric wasn’t explained at the time. However, in August, de Blasio described it as “the toughest in the world,” comparing it to the World Health Organization’s 5%.

Calculations for assessing viral spread differ between health organizations and universities, and benchmarks vary from community to community and state to state. 

For example, the Centers for Disease Control and Prevention says when a percentage of tests in the community that are positive over the last 14 days is under 3%, schools are operating in the lowest of five categories of risk. 

The CDC guidance features several other measures, rather than focusing on a single metric, and it does not recommend closing schools if the positivity rate reaches a certain level. Rather, reaching 3% up to 5% simply moves schools from the category of “Lowest risk of transmission in schools” to “Lower risk of transmission in schools.” 

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The phrase “3%, based on a seven-day rolling average” has become familiar. But where did this metric come from, and what science is it based on?

One may wonder, then, why the closure of New York City schools is based on this specific benchmark. 

On July 31, when he announced the plan, de Blasio said, “we’ve made it an absolute cardinal rule that all these specific plans that have been talked through with the teachers’ union constantly.” 

A couple weeks later the Department of Education released a statement: “We engage our union partners every day, and have been discussing policy details for weeks. The CSA and UFT know we’ll only open our doors if we meet the strictest standards set by any school district in the nation.”

More recently, the mayor has explained the origin somewhat differently. 

“I want to make clear that this is something the city decided, it’s not part of any collective bargaining agreement,” he said on Friday. When pressed on the matter, he specified that it was “not a proposal from the unions.” 

Instead, he described the benchmark as “a social contract.”  

“The standard was based on what we understood at the time to be the numbers that would indicate that we are reaching a bigger problem,” he said. “It was our health care leadership working with me to determine what we thought was a standard at that time.” 

When he announced the plan, de Blasio said, “we’ve made it an absolute cardinal rule that all these specific plans that have been talked through with the teachers’ union constantly.” More recently, the mayor has explained the origin somewhat differently. 

As the city has approached the verge of shutting down, some people, including Gov. Andrew Cuomo, have questioned its use, suggesting that the city should focus on the positivity rate in schools themselves, not in the city overall.

And, he said, that schools should close only if 3% of staff and students test positive. The current positivity rate within city schools is 0.16%. “The problem is not coming from the schools, it’s coming from the bars, the restaurants, the gyms and the living room family spread,” Cuomo said. 

Jesse McKinley, for the New York Times, reported that Cuomo said he had “the authority to order” de Blasio to keep the schools open, but that the governor gave no indication he planned on exercising that power. And de Blasio has thus far resisted any such suggestions. 

On Friday, the mayor acknowledged that care for COVID-19 has improved since the decision was made to adopt the threshold in July. However, “we asked everyone to trust in it. And part of keeping trust is staying consistent.” 

On Saturday, a small rally in Foley Square, in downtown Manhattan, arose protesting the possibility of schools closing. “The rate that the mayor agreed to in July,” Karen Vaites, a parent, said, “is out of touch at this point.” 

Earlier Sunday, the city told Chalkbeat that schools would remain open on Monday but that it was not considering any changes to the shutdown plan. 

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These may be savvy political moves, and one that a mayor or governor feels he needs to make. But neither is following the science.

So, essentially, the mayor admits that the threshold was created during a different time, when the perceived and real risks were different from what they are now, and before we gained evidence that schools would have a lower positivity rate than the surrounding community. 

He says he is following the science. Yet because of a supposed social contract, despite the changing evidence and circumstances, he will not revise the threshold. He’s sticking with a benchmark crafted in July apparently only because it was something he decided was an agreement. 

The governor, who also often says he follows the science, suggested an alternative calculation for determining whether it’s safe to keep schools open, and could force de Blasio to do so, but, as of yet, has not taken action. 

These may be savvy political moves, and one that a mayor or governor believes he needs to make. But neither is following the science.

ABOUT THE AUTHOR

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David Zweig

Zweig writes about technology and culture for Wired, the New York Times, and The Atlantic. You can follow him at @davidzweig