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 released 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 percent fewer cases of in-school transmission of COVID-19 when compared to schools with optional or partial masking policies,” according to the study.

The delta variant surged in August 2021, just as schools welcomed 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.

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 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.

For the first time since schools reopened during the COVID-19 pandemic, California is lifting its statewide mask mandate on March 12 for K-9 students. However, the California Department of Public Health (CDPH) is “strongly recommending” that students and teachers continue to wear masks indoors.

Bay Area county health departments and local school districts still have the authority to uphold mandatory masking requirements for schools.

The CDPH wrote, “The following guidance is designed to keep California K-12 schools open for in-person instruction safely during the COVID-19 pandemic, consistent with the current scientific evidence. The foundational principles are ensuring access to safe and full in-person instruction for all students and keeping equity at the core of all efforts. In-person schooling is critical to the mental and physical health and development of our students.”