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SAN FRANCISCO, Calif. (KRON) — Some skeptical parents and anti-mask advocacy groups questioned the necessity of forcing students and teachers to wear masks for the majority of the 2021-22 school year. Did masks really help slow the spread of COVID in classrooms?

A study funded by the National Institutes of Health and released on Thursday found that mandatory masking during the delta variant surge was critical for slowing down the transmission of infections.

“Schools with mandatory masking during the Delta surge had approximately 72% fewer cases of in-school transmission of COVID-19 when compared to schools with optional or partial masking policies,” according to the new study.

The delta variant surged in August 2021 just as schools were welcoming back students for the new school year.

The study included more than 1.1 million students and over 157,000 staff members attending in-person learning at schools across nine states: California, Texas, North Carolina, Wisconsin, Missouri, Washington, Georgia, Tennessee, and Kansas.

Researchers said the study provides strong evidence that “masking remains a critical preventive measure during variant-triggered surges of high community infection rates.”

The study included 61 school districts, kindergarten through grade 12, with data from July 26, 2021, through Dec. 13, 2021, a period encompassing the delta surge.

A second-grade student wears a Hello Kitty style mask in class. (Photo by INA FASSBENDER/AFP via Getty Images)

According to the Centers for Disease Control and Prevention, the virus is transmitted by breathing in droplets and small particles from an infected person or touching surfaces that have droplets containing the virus on them.

Most COVID cases among students and staff were acquired from the community, and 10% of cases were acquired within schools.

“For every 100 community-acquired cases, school districts with mandatory masking had approximately 7.3 cases of in-school infections, while optionally masked districts had 26.4 cases of in-school infections,” researchers wrote.