SAN FRANCISCO, Calif. (KRON) — 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.

Some skeptical parents and anti-mask advocacy groups questioned the necessity of forcing students and teachers to wear masks for the majority of the 2021-2022 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 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 new study.

The Delta variant surged in August of 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)

The virus is transmitted primarily by inhalation of respiratory aerosols, according to the California Department of Public Health.

Most COVID cases among students and staff were acquired from the community, and 10 percent 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.

With the omicron variant surge fading, California is lifting the state’s indoor mask mandate for K-12 schools after March 11. Instead, the California Department of Public Health is “strongly recommending” that students and teachers continue to wear masks indoors.

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

To read more about the CDPH’s new school mask guidance, click here.

“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,” the CDPH wrote.