A prominent group of Washington disease modelers is attempting to close in on answers to big questions about how the coronavirus spreads between schools and the community, but is facing pushback from a handful of researchers who say the group’s mathematical modeling should be tested against real-world results. 

Washington has logged at least 122 coronavirus outbreaks in schools since the start of the pandemic.

In new research released Wednesday, modelers at the Seattle-based Institute for Disease Modeling (IDM) found that the number of infected people who show up to school is proportional to spread in the community. More involved or costly safety measures — like vaccinating teachers and coronavirus testing at schools — may offer limited protective benefit when basic protocols like masking and distancing are in place, they say, but could be key to keeping case counts down if more transmissible coronavirus variants become common. Like the group’s previous work, this study is unpublished and has not been peer reviewed.

The conclusions don’t significantly change the conversation about coronavirus and schools. But, “Now we can really say how much and say precisely why [community transmission] matters,” said Dan Klein, who led the study and is senior research manager at IDM. “If you have twice as much COVID out in the community, COVID shows up at schools twice as often.” The modeling group is part of the Bill & Melinda Gates Foundation and has assisted the state Department of Health (DOH). 

The findings broadly reaffirm what other studies have found, say officials at DOH, and will help the agency target costly safety strategies, like frequent surveillance testing, to schools that need it most. It’s expensive and takes time to repeat testing, “so they probably are most valuable if they are done in areas with high disease [levels],” said Lacy Fehrenbach, one of the state’s top health officials. 

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Evidence is mounting that schools aren’t superspreaders. And the U.S. Centers for Disease Control and Prevention (CDC) has released a blueprint for how schools can reopen safely. But cases do crop up, and in Washington, at least 305 cases are associated with outbreaks in schools. An outbreak is defined as two or more linked cases over a 14-day period.

The new research suggests that while schools will inevitably have cases, many don’t seed outbreaks. When the virus does spread between people at school, the outbreak usually only involves a handful of people, the researchers found.

This generally fits with what schools have seen in practice, said Dan Goldhaber, director of the University of Washington’s Center for Education Data & Research, who has studied the effect of in-person learning on community transmission, and was not involved in the IDM research.

But the IDM researchers, who are using data to simulate findings, could strengthen their work if they tested their model against real-world results, he said. Several districts have returned for in-person learning and are reporting case counts publicly. “I don’t understand why they don’t test the model empirically,” based on observed data, Goldhaber said. 

Judith Malmgren, an epidemiologist affiliated with the UW, went further. Of the new study, she said, “I don’t think it’s useful.” 

“If you are going to look at what’s going on, we need to look at what’s actually happening,” she said.

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In response, Klein said the state’s reporting on school outbreaks doesn’t include enough data to check against the model. 

In its previous work, the IDM team used King County demographic and infection data at a particular point in time to simulate various school reopening scenarios. This method wasn’t generalizable, however, since transmission patterns and other aspects of the pandemic are constantly in flux. 

This time, the researchers used a revised computer algorithm to model three points on the transmission chain: prevalence in the community and its effect on COVID-19 introduction into the classroom; factors that determine the size of a school outbreak; and the role of schools in driving community spread. 

Klein and his colleagues found that how often coronavirus is introduced into classrooms is proportional to community spread. For instance, every .1% increase in prevalence in the community results in an additional case every 2 months or so in a 500-person school. A bigger school would have more cases. This finding meshes with prior work suggesting that what’s happening outside school walls influences case rates inside them. 

Schools can only modestly control how many people come to school infected, the researchers found, through efforts like surveillance screening. Cases are also less likely to pop up at elementary schools than at high schools.

Washington requires masks, social distancing and other measures in schools that reopen. Once someone inside a school is infected, the researchers found, the spread of the virus depends on the school’s ability to perform these and a host of other safety measures, such as contact tracing, symptom screening and diagnostic testing. 

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The findings point to a chain reaction in transmission: The more infected people who show up to school — and the more efficiently the virus spreads there — the more common it is for school-related infections to spur community spread. Although the researchers continue to use King County data, they say their results are likely applicable to other corners of the state since they tested and found consistent results using data from Spokane, Franklin and other counties.

The study has several limitations. It doesn’t capture all the ways that students and staff catch or transmit the virus, such as on school buses, at day-care centers or during after-school sports. And the researchers made several assumptions about children’s susceptibility to getting infected. 

Malmgren and Goldhaber were quick to identify other concerns with the research. A big one: The work doesn’t test against real community or school data, they said, such as differences in infection rates between students and teachers.

Some limited data on who gets infected is now available. Statewide data suggests that teachers and staff make up 50% of those infected in school outbreaks, though there are far fewer adults than students in school building. And a CDC report from this week on a school district in Georgia found that teachers play a central role in school transmission.

“That’s really important because the question now is, if we open schools, how are we going to protect teachers? And should we vaccinate teachers?” Malmgren said.

The study also mischaracterizes how often infections become outbreaks, Malmgren said. According to a recent DOH report that includes data through December 2020, a majority of outbreaks in Washington schools involve two to three cases; in all, the state recorded 84 outbreaks while about 200,000 students were being served in person. State officials have provided little context for what this data implies about transmission in schools. But on Wednesday, state health official Fehrenbach said, “The question of how many outbreaks or introductions, are they significant? Is it a lot? That’s a great question.”

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Said Klein: “The number of outbreaks has been relatively lower than expected, but we do not mean to trivialize them, or minimize the risk.”

Goldhaber said he would liked to have seen the researchers’ model transmission patterns for scenarios where children aren’t learning in-person — such as students learning at home, or getting together in small learning pods. 

“What they don’t model is what happens if kids are not in school and that, for community spread, matters,” he said.