The Centers for Disease Control and Prevention on Friday offered more evidence that school mask requirements can help keep children healthy and in classrooms, showing lower spikes in pediatric coronavirus cases and fewer school outbreaks in places that require them.

In an analysis of 520 U.S. counties, the CDC found that pediatric cases rose more sharply in places without school mask requirements. And in a separate report that looked at Arizona’s two most populous counties, the agency found that schools without mask requirements were 3.5 times as likely to experience an outbreak than schools with them.

Though polls show that a majority of parents support mask requirements — and despite recommendations from pediatricians and the CDC — schools remain bitterly divided over whether to implement them. Opponents of mask mandates say parents should get to decide whether their children wear them.

The pandemic has already taken a toll on the new school year. In a third report, the CDC said more than 900,000 students in 44 states had been affected by closures between Aug. 1 and mid-September. Many schools are contending with challenges on multiple fronts, including outbreaks that force staff to quarantine and staff shortages that make it difficult to find replacements.

The CDC looked at pediatric case data for about 17% of U.S. counties and found that, on average, pediatric cases rose after schools reopened. But counties without mask requirements saw larger increases — about 18 cases per 100,000 more — than those with them. Among the counties the CDC studied, a majority — about 62% — did not have school mask mandates.

Because the data was limited to so few counties, and because it reflected all pediatric cases — and not just those of school-aged children — the agency said that “the results may not be generalizable.” Still, the agency said: “School mask requirements, along with other prevention strategies, including COVID-19 vaccination, are critical to reduce the spread of COVID-19 in schools.”

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There is abundant evidence supporting the efficacy of masks in reducing the spread of the coronavirus, including with children and within schools. In July, the CDC and the American Academy of Pediatrics recommended universal masking in schools to ward off the more contagious delta variant. In North Carolina, researchers closely tracking 100 school districts with mask mandates from March to June found very little transmission in schools. A case study in Marin County, Calif., where an unvaccinated teacher who pulled down her mask to read to her class infected half of her students, provides evidence of the contrary.

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But opposition to mask mandates is still strong, and the issue remains deeply polarizing in parts of the country. In several states, GOP leadership has barred school districts from requiring masks, sparking court battles. School board meetings have become battlegrounds, drawing protesters and angry parents. This week, the national associations representing school board members and superintendents issued a statement expressing concern over reports that its members were being harassed and threatened.

“We are concerned with the increasing reports of our members — school superintendents and school board members — who are working to ensure a safe reopening of schools while addressing threats and violence,” the statement said. “School leaders across the country are facing threats because they are simply trying to follow the health and scientific safety guidance issued by federal, state, and local health policy experts.”

The resurgence of the coronavirus has meant many students have been shuffling in and out of quarantines, or forced to return to online learning because of acute staff shortages. It has eroded hope that this school year would represent a turning point for pandemic schooling, which for many has been chaotic, isolating and inadequate.

Pragya Upreti, 17-year-old senior at Lafayette High in Lexington, Ky., testified before lawmakers in the GOP-led statehouse last month, urging them to leave the governor’s mask mandate in place. Her high school has had one since the beginning of the school year, and she said going to a school where no one was masked would make her “fearful.”

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“We really don’t want to go back to a virtual setting of learning,” Upreti, a member of Kentucky Student Voice, said in an interview. “Mask mandates keep schools open.”

The CDC’s analyses are the first to look at COVID in schools in the era of the delta variant, which has proved to be more contagious than its predecessors and driven pediatric infections to record highs. The agency is also is the first to compare schools with mask mandates to schools without them. Most research has focused on the former.

Danny Benjamin is a pediatrics professor at Duke University who co-led the North Carolina study. He said the new CDC report, despite its limitations, represents an “a meaningful contribution” to the existing body of research.

“It’s the first publication with the delta variant in American schools that compares schools with and without a mask policy,” Benjamin said.

But he added that the question of whether students should wear masks in schools should be settled by now.

“To me, the question as a researcher is answered,” Benjamin said.

Emily Oster, a Brown University economics professor, has spent the duration of the pandemic collecting and analyzing school data. She was not confident that the new data would convince those who do not believe in masks.

“For the people that don’t support mask mandates in schools, they won’t be convinced by this,” Oster said. “There is such polarization.”