(HealthDay)—Wearing masks in schools appears to sharply curtail the spread of COVID-19, despite the dominance of the highly contagious Delta variant, two new U.S. studies show.

Published by the U.S. Centers for Disease Control and Prevention, the reports found there were fewer outbreaks in schools that required masks.

In the first study, researchers focused on students in two Arizona counties, Pima and Maricopa, the homes of Tucson and Phoenix. The second study looked at schools in 520 counties throughout the United States.

“The school year starts very early in Arizona, in mid-July, so we had the advantage of being able to get an early look at data for the new school year a bit sooner than was possible for the rest of the country, which was important, because of the transmission of the Delta variant,” study co-author J. Mac McCullough, an associate professor at Arizona State University, told The New York Times.

The researchers found that only 21% of the 1,000 public schools in those Arizona counties implemented a universal mask mandate when they opened, with about half having no mask requirement of any kind. Another roughly 30% began requiring masks 15 days after school started.

The data showed there were 191 school-associated COVID-19 outbreaks about a week after schools opened.

About 60% of those outbreaks happened in schools with no mask requirement, the report found. That compares to about one-third of outbreaks in schools that began their mandates after school had already started and 8% in schools that began the year with mask requirements regardless of vaccination status. That means schools with no requirements were 3.5 times more likely to have an outbreak.

To be considered an outbreak by the study required that two or more confirmed positive cases happened among staff or students in a two-week period. Masking, distancing, staying home when sick and getting vaccinated when eligible are all actions that can help prevent school-based outbreaks, according to the CDC.

“This study really shines a lens on the masking part of that,” McCullough said.

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