Face-mask requirements in schools drastically reduce the spread of Covid-19 among children, two studies published by the Centers for Disease Control and Prevention suggest.

Researchers in Arizona found that schools that didn’t require masking at the start of the academic year were much more likely to experience Covid-19 outbreaks than those that did. A second study found that counties without mask requirements for schools saw larger increases in pediatric Covid-19 case rates compared with counties with school mask requirements. ...

Face-mask requirements in schools drastically reduce the spread of Covid-19 among children, two studies published by the Centers for Disease Control and Prevention suggest.

Researchers in Arizona found that schools that didn’t require masking at the start of the academic year were much more likely to experience Covid-19 outbreaks than those that did. A second study found that counties without mask requirements for schools saw larger increases in pediatric Covid-19 case rates compared with counties with school mask requirements. 

The CDC recommends universal indoor masking in schools for both students and staff, regardless of vaccination status. The agency and many public-health experts are pushing a so-called layered approach that includes not only masking, but also social distancing, screening testing, improved ventilation and handwashing.

In the Arizona study, researchers looked at two of the state’s most populous counties, where students started in-person learning in late July and early August. The researchers tracked which schools required students and staff to wear masks indoors and which ones didn’t, and they also noted when those mask mandates were implemented—right at the beginning of the school year or sometime after classes began.

The researchers then studied a database where all school-associated outbreaks in the state were required to be reported and counted 191 school outbreaks between July and August. Around 8% were in schools that implemented early masking requirements, 33% were in schools that required masking later on, and just over 59% were in schools without masking requirements. Schools without mask requirements were 3.5 times more likely to have an outbreak of Covid-19 than those that implemented masking mandates right at the start of the school year, the researchers found.

“Lapses in universal masking contribute to COVID-19 outbreaks in school settings,” the researchers wrote in a report published on Friday.

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In another study, researchers at the CDC looked at pediatric Covid-19 case rates in 520 counties, some with and some without a masking requirement. After taking into account factors such as age, race, pediatric vaccination rate and rates of community transmission, they found that counties with school mask requirements had smaller increases in their rate of pediatric Covid-19 cases than those without those requirements.

“Masks work,” said Mobeen Rathore, chief of infectious diseases at Wolfson Children’s Hospital in Jacksonville, Fla., who wasn’t involved in the studies.

He said the research reflected what he has seen in his own community.

In Duval County, where Wolfson is located, there were no masking requirements for schools when classes reopened in early August, he said. Data from the Duval County Public Schools Covid-19 Case Dashboard show that cases among students hit a high in late August. Then, on Sept. 7, the county school board approved a 90-day mask mandate for all students.

“After that, the number of cases started to go down in the school system,” said Dr. Rathore. Cases among students have been on a downward trajectory since then, according to the dashboard.

Write to Sarah Toy at sarah.toy@wsj.com