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Two new studies on whether to keep schools open during the coronavirus pandemic come to strikingly similar conclusions: it’s not a simple yes or no. Instead, there are public health thresholds that can indicate when in-person classes are safe. 

The similarity in the results is striking considering that the research teams used different data and took different approaches to crunching the numbers.

“The fact that it seems safe [to open schools] in some places but perhaps not in others isn’t surprising,” said Tulane University economist Douglas Harris, a researcher on one of the studies. “Schools should spread the virus less in places where there is less of it to spread.”

Harris’s study, released on Jan. 4, looked at every 2020 school opening in the country through the fall and tracked how many people in each county landed in the hospital because of COVID-19 for the following six weeks. Harris and two Tulane health researchers found that school openings didn’t add to the number of people in the hospital, as long as the COVID-19 hospitalization rate was below 36 to 44 people per 100,000 residents per week before schools reopened. 

The study took place before the recent surge. Only 58 percent of U.S. counties were still below this threshold in December and 42 percent exceeded it. During the months of the study, only 25 percent of counties were above the threshold and the researchers found conflicting results for what happened in these counties. There was some evidence that open schools were exacerbating COVID-19 hospitalization rates and some evidence that they weren’t.  

“It was inconclusive,” said Harris. “When we look at below the 75th [percentile], we never see any evidence of an increase in hospitalizations. So that’s why we’re confident in that conclusion.”

Harris characterizes 36 to 44 hospitalizations per 100,0000 people as a “rough guideline” for keeping schools open. “I wouldn’t say once you’re at 45 hospitalizations per 100,000, which is just above our threshold, that suddenly you’re at risk,” he said. 

The second study, released on Dec. 20, 2020, focused on Michigan and Washington during September, October and November. Instead of hospitalizations, the researchers looked at the number of people who tested positive for the coronavirus. Specifically, they calculated how the number of COVID-19 cases in the community changed if school was in-person, remote or a hybrid of the two. At first glance, the researchers saw strong increases in COVID-19 rates associated with in-person school. But once the researchers factored in other explanations, such as mask wearing, the association between in-person school and COVID-19 cases usually disappeared. 

Two of the biggest factors affecting the monthly increase in COVID-19 cases turned out to be a community’s general adherence to mask wearing, as measured by a public health survey, and what percentage of the community voted for Donald Trump in 2016 —  not whether schools were open.

“At low levels of pre-existing cases, we don’t find evidence that having the schools open is leading to additional COVID cases,” said Dan Goldhaber, a vice president at the American Institutes for Research, a nonprofit research organization, and one of the seven authors on the Michigan-Washington study. “That doesn’t mean that there is not transmission in the schools. I think it’s almost assuredly the case that there is transmission in schools.”

But in-person school exacerbated community spread once COVID-19 rates exceeded certain thresholds. In Washington, that threshold was low, only 5 cases per 100,000 people. In Michigan, that threshold was much higher at 21 cases per 100,000 people. 

The researchers puzzled over why the threshold for safely operating schools would be different in the two states. One possible explanation is that community compliance with public health directives was generally stronger in Washington than in Michigan. So when schools closed and students learned remotely in Washington, families were more likely to limit social contacts. In Michigan, schools may have been adhering to stricter public health guidelines than the broader community. Schools were a safer place, even as the pandemic surged.

“If kids are not in school, maybe they are in childcare or they’re hanging out with their friends or doing other things that also lead to spread,” said Goldhaber.

The researchers didn’t detect any threshold for hybrid instruction, when students might only attend in-person school two days a week and learn remotely the remaining days. No matter how high the virus rate, they didn’t see evidence that limited school attendance increased community spread. In Washington, hybrid and in-person were lumped together and there was no distinction between the two. 

A big caveat in both studies is that in-person school doesn’t mean full capacity. In many communities, half the families chose to continue learning remotely, making in-person schooling less dense and much safer. It is unclear from this research how in-person school at full capacity would affect community spread of the virus.

A similar study in Germany found that school reopening reduced coronavirus transmission because schools quarantined students who tested positive. This threat, combined with strong messages sent by educators to encourage safe behavior, may have led students to also be more careful in social distancing outside of school.  

Coming to a consensus on school openings isn’t easy. In November 2020, Brown University economist Emily Oster made a quantitative argument that there weren’t signs of COVID-19 outbreaks inside schools and that schools should remain open or reopen. But Oster’s data comes from schools that had voluntarily agreed to share data with her. Schools with outbreaks might not be keen to share that information. Both of the newer studies take advantage of administrative public health data that is less biased. 

When I asked Oster her reaction to the latest research, she pointed out that both studies found that in-person school generally didn’t make the pandemic worse until you got to the edges of the data, where statisticians can reasonably argue about interpretations. Oster remains unconvinced that there are thresholds or tipping points above which it’s unsafe to operate schools.

 “I think it is never a good idea to say there’s some number like 12, or 15, or 45, above which is dangerous and below which is perfectly safe,” said Oster. “I think it certainly is possible, and probably likely, that as the rates go up more, it becomes more difficult to keep schools open. That may be because we worry about spread, and maybe because of staffing, and maybe for a bunch of reasons.”

Katharine Strunk, one of the authors of the Michigan-Washington study, agrees that the specific numbers in either paper shouldn’t be grasped too tightly. Strunk says the lesson from both papers is that schools can open safely when COVID-19 rates are at “moderate” levels. “I would work really closely with local health officials to understand what moderate means,” said Strunk, a professor of education policy at Michigan State University. “It’s not one number. Even if COVID levels are high, but the growth rate isn’t, maybe it’s not spreading. The local health officials are the ones who should be working with the district to help them think through whether or not it’s safe to reopen or stay open.” 

This story about in-person school and community spread was written by Jill Barshay and produced by The Hechinger Report, a nonprofit, independent news organization focused on inequality and innovation in education. Sign up for the Hechinger newsletter.

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Jill Barshay writes the weekly “Proof Points” column about education research and data, covering a range of topics from early childhood to higher education. She taught algebra to ninth-graders for...

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  1. Thank you for sharing the interesting results of these two studies. Our team has been closely following this as well and have found that while reopening schools is never a “zero-risk” decision, incorporating public health guidance can minimize risk for students and staff.

    Our team has launched the Parabola Project (minimizing risk while maximizing learning and wellness), which brings together healthcare and education experts to help guide school leaders with reopening resources. We have come up with nine public health principles, which outline safety measures that educators can implement, along with toolkits, resources, ideas, and strategies. We’d love to share these free resources with any interested educators, as well as hear from school leaders and teachers about unique approaches to school reopening to showcase. Our resources can be found at parabolaproject.org. We’d love to keep in touch!

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