U.S. schools requiring masks have experienced fewer outbreaks, the CDC found.

School mask warrants have sparked controversy in many parts of the country. Now, two studies, released by the Centers for Disease Control and Prevention on Friday, provide further evidence that the masks protect children against the coronavirus, even when community rates are high and the contagious Delta variant is circulating.

A study, conducted in Arizona, where children returned to school in July, found that schools that did not require staff and students to wear masks were 3.5 times more likely to have an outbreak of viruses than schools that required universal masking.

A second study looked at infections in all children in 520 different counties in the United States and found that once the public school year began, pediatric cases increased at a much higher rate in counties where schools did not. did not need masks.

The first study analyzed data from about 1,000 public schools in Maricopa and Pima counties, which include the metropolitan areas of Phoenix and Tucson, and represent the bulk of the state’s population.

Only 21% of schools implemented a universal mask mandate upon opening, and almost half had no mask requirement at all. About 30 percent adopted a mask requirement around 15 days after starting school.

Between July 15 and August 31, 191 school-associated virus outbreaks occurred about a week after school started. The majority of them - 113 outbreaks, or nearly 60% of the total - occurred in schools without a mask requirement.

Only 16 outbreaks, or 8% of the total, have occurred in schools that have put in place mask requirements regardless of vaccination status from the start. There have been 62 outbreaks, or about a third of the total amount, in schools that implemented a mask requirement after the start of the school year.

The study defined an outbreak as two or more confirmed positive infections among staff or students over a 14-day period.

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

The CDC recommends a layered approach to preventing coronavirus outbreaks in schools - masking, distancing, staying home when sick and getting vaccinated for those who qualify. “This study really shines a light on the masking part of that,” Dr. McCullough said.

The second study examined the association between school mask policies in a given county and community-wide infections in children, finding that counties without a school mask requirement experienced a larger increase in pediatric cases after starting school than counties with school mask requirements.

Between the week before school started and the second week of school, the number of pediatric infections increased by 35 cases per 100,000 in counties without a mask requirement, while the number increased by 16 cases for 100,000 inhabitants in the counties with a requirement for a school mask.