School districts with mandatory masking had 7.3 cases of in-school infection for every 100 community-acquired cases. Districts with optional masking had 26.4 cases of in-school infection for every 100 community-acquired cases. The results were similar when the researchers adjusted for district size and weeks of reporting data.

This study was conducted when Delta was the dominant variant. The team didn’t obtain data on how school masking affected the spread of Omicron. However, based on all available data, the CDC recommends indoor masking whenever there are high community levels of COVID-19.

“These findings show that school masking remains a critical preventive measure in times with high community infection rates, as we observed with Omicron, or if a variant emerges that escapes immunity,” Benjamin says.

Source: Mandatory masking in schools reduced COVID-19 cases | National Institutes of Health (NIH)