As summer wore on and COVID Delta variant cases surged, Megan Jehn, a professor of epidemiology at Arizona State University, was getting calls from concerned parents, teachers, and administrators worried about in-person school and about the lack of masking policies within some districts.
Jehn leads a contact tracing team at the ASU that investigates the majority of COVID cases in Maricopa County. The school year starts early in Arizona, and when students began returning to class in July, Jehn and her team noticed an unsurprising trend.
“We'll investigate a couple of hundred cases a day, and with the pediatric cases, we were asking them about masking in schools, and the majority of all new school outbreaks that we were investigating, were coming from unmasked children or unmasked schools,” Jehn says.
However, these observations were anecdotal and Jehn wondered what would happen if they studied school outbreaks more systematically. Arizona’s early return to school meant new data on how the spread of Delta was impacted by school mask policies could be gathered in time to potentially influence masking policies at school districts throughout the country.
What Jehn and her co-authors found in their research, recently published by the CDC, was that schools without mask requirements were about 3.5 times more likely to have COVID-19 outbreaks than schools that started the school year with mask requirements.
How The Research Was Conducted
Jehn and the researchers she worked with looked at data from about 1,000 K-12 schools in Maricopa and Pima Counties, which together are home to 75 percent of the state’s student population. Between mid-July and the end of August, they identified 191 school-associated COVID-19 outbreaks -- defined as the occurrence of two or more laboratory-confirmed COVID-19 cases among students or staff members at the school within a 14-day period -- at least a week after school started.
The schools studied were divided into three categories: schools that started the year with a mask requirement for all students and staff in place regardless of vaccination status; schools that adopted a mask requirement after the school year started; and schools that did not have a mask requirement.
Twenty-one percent of the schools studied started the school year with a mask requirement, however, these schools accounted for just 8.4 percent of the outbreaks. Meanwhile, 48 percent of schools had no mask requirements, which accounted for 59.2 percent of the outbreaks. Schools with late mask requirements accounted for 32.5 percent of the outbreaks.
The clear advantage for schools with mask requirements was found even after researchers adjusted for confounding variables.
“We adjusted for the factors that we know are likely to drive school outbreaks,” Jehn says. “So, how much transmission is happening in the community around the schools. We looked at the socioeconomic status of the school, we looked at the size of the school, and then we looked at the ages of the kids attending the school.”
Prior to these adjustments, COVID-19 outbreaks were deemed to be 3.7 times more likely in schools without mask requirements, but controlling for confounders brought that estimate down to 3.5 times more likely.
Takeaways for Teachers and Parents
Despite the edge schools with mask requirements had, Jehn does not want parents or teachers who read about her research to get the wrong idea. “I don't want them to walk away with the feeling that schools are inherently unsafe, because they're not. The biggest driver of a child contracting COVID is the transmission in the community around the school,” she says. Proper mitigation can help prevent cases that are caught in the community from spreading within schools.
The study strongly suggests that mask policies can be effective even without 100 percent compliance.
“We didn't have people at the schools walking around trying to systematically observe how many people had a mask on and what kind of mask they were wearing, we just looked at whether policies were in place,” she says. “We know that people are not always compliant, we know the parents opt-out. And we also know that in those schools that don't have masking requirements, some kids do mask.”
She adds, “If you have kids in the masked schools who are not compliant and you have kids in the unmasked schools who are wearing masks, it makes it harder to find that association. So the fact that we actually did means that the true effect is probably much greater than what we've reported.”
However, laypeople often misunderstand this. “They say, ‘Well, you know, your results can't be reliable because we know that this behavior isn't compliant all the time.’ But actually, I think that's a strength of our study,” Jehn says. “All of these schools that had mask requirements, they allowed opt-outs. We know that kids aren't perfectly compliant all of the time. But even with all of those things, only 8 percent of all the school outbreaks that we investigated came from a school with a masking requirement.”