Overall, schools that required masks had 23% fewer Covid-19 cases on average than those that didn’t.
“This is a valuable finding consistent with other past data during the Delta wave,” Kristen Nordlund, a CDC spokesperson, said in an emailed statement.
The research compared Covid-19 infections in 233 public school districts. Among these, 30% required students and staff to mask up, 21% required masks part of the time, and 48% didn’t have mask requirements.
Mask policies had the most effect for older students and in combination with higher vaccination coverage.
Compared with districts that didn’t require masks, districts that did had 22% fewer Covid-19 cases among students in kindergarten through fifth grade; 31% fewer cases among students in grades six through eight; and 32% fewer cases among students in grades nine through 12.
Overall, vaccination coverage for students and staff ranged from 13.5% at the start of the study to 18.6% by the end.
Districts with higher vaccination coverage — with more than 40% of students and staff vaccinated — combined with mask requirements, had 38% fewer Covid-19 cases compared with districts without mask requirements.
The authors note that the study has some limitations. Researchers didn’t have information on ventilation or other community or school-based prevention efforts that could have also influenced the results. They weren’t able to tell how well people adhered to the masking rules in each district. And quarantine rules after students tested positive differed between schools with masked and unmasked students.
In accordance with CDC guidelines, students were less likely to be quarantined if they were wearing a mask while in close contact with a student who tested positive, a policy that researchers say may have made masks look less effective than they actually were.
The data was collected before the Omicron variant became prevalent in the United States.
The study is being released just a week after the CDC made consequential adjustments to how it measures levels of Covid-19 within communities.
Instead of looking solely to case counts and rates of test positivity, the agency is now advising state and local officials to base their Covid-19 mitigation measures on hospitalization rates and local hospital capacity, in addition to new case rates.
A CNN analysis of the data finds that 7% of the US population is now in counties with “high” Covid-19 community levels, where even healthy people are advised to wear masks.
“As Omicron has become the predominant variant, COVID-19 levels have shifted nationally and CDC no longer recommends universal masking in all schools. Instead, CDC recommends masking in schools in areas with high COVID-19 Community Levels. CDC will continue to follow the data and science and ensure our recommendations meet the moment we’re in,” Nordlund said in the statement.