A US study shows that coronavirus infections among students and staff increased significantly at schools where mandatory masking requirements were lifted.
A research group including scientists at the Harvard School of Public Health has published its analysis in The New England Journal of Medicine.
The study focused on schools in and around Boston in the eastern state of Massachusetts.
It compared 70 school districts that lifted masking requirements in February this year with two other districts that kept the requirements in place because they tended to have more students per classroom.
The sample included about 340,000 students and school staff.
The group found that in the districts that lifted masking requirements, the number of cases per 1000 students and staff reached 134.4 during the 15 weeks that followed. But the number of cases stayed at 66.1 in the districts that continued masking requirements.
Its statistical analysis showed that the lifting of masking requirements was associated with an additional 44.9 COVID-19 cases per 1,000 students and staff.
The scientists say the estimate corresponded to an additional 11,901 cases and accounted for about 30 percent of all infections among students and staff.
They say there is no clear existing evidence that masking inhibits learning or harms development.
The researchers suggest that masking is an important strategy for lowering transmission and reducing student absences during periods of high community transmission.