Researchers at the C.D.C. and the Georgia Department of Public Health surveyed 169 elementary schools in Georgia at the end of 2020, after in-person learning had resumed in the state.

Schools that improved ventilation had 39 percent fewer Covid cases, compared with schools that had not. Schools that combined better ventilation with filtration had 48 percent fewer cases.

A large study of schools in Italy estimated that students in classrooms equipped with ventilation systems or devices that deliver clean air had an at least 74 percent lower risk of infection than students in classrooms with open windows.