School mask mandates in the U.S. reduced coronavirus transmission

Mandatory masking lowered transmission of the delta variant by 72 percent

kindergartners in a classroom, wearing masks

Kindergarten students at Stanley Mosk Elementary School in Los Angeles wear masks while in class on March 11.

David Crane/MediaNews Group/Los Angeles Daily News via Getty Images

The verdict is, once again, in: Masking in schools is effective.

During the COVID-19 pandemic’s delta variant wave, schools that required masking had approximately one-fourth the rates of in-school coronavirus transmission than schools with optional or partial masking policies, researchers report online March 9 in Pediatrics.

While the findings reinforce past research on masking efficacy, this is the first study to focus on secondary transmissions, or transmissions that happen at school, and how the rate is impacted by vaccination, community spread and district size, says study coauthor Danny Benjamin, an epidemiologist at Duke University. The team found that schools play a small role in terms of transmission in a community, accounting for less than 10 percent of total cases in the study. But masks are what help keep that number low.

To conduct a wide-reaching study, the researchers reached out to every kindergarten through 12th grade public school district in the United States — more than 13,800 of them — to track their case numbers and origins from July to December 2021. The 61 districts that answered the call consisted of more than 1 million students and nearly 160,000 staff across nine states.

Now that vaccines are available for kids ages 5 to 11, the researchers are collecting more data from schools. Masking has continued to work during the omicron variant surge, but even short intervals of mask removal, such as at lunchtime, result in much higher transmission than during the delta wave, Benjamin says.

As state mask mandates lift, the onus for mask requirements in schools falls to the school districts. To help school administrators decide if and when students need to mask, Benjamin and colleagues developed the Masking and Mitigation Considerations Calculator. The tool allows administrators to input their community COVID-19 rates and determine how masking decisions in schools will affect case numbers.

“This is a very granular and local and nuanced decision,” Benjamin says. Right now, as the omicron surge recedes in some parts of the country, “it’s a reasonable discussion to be having.”

Anna Gibbs is the spring 2022 science writing intern at Science News. She holds a B.A. in English from Harvard College.

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