Chicago will require masks in school this fall, regardless of vaccination status.

The Centers for Disease Control and Prevention issued new guidance earlier this month that called for a full return to classrooms in the fall and recommended that masks be optional for fully vaccinated students and staff.

But the guidance left a lot of details up to state and local governments, advising districts to use local coronavirus data to guide decisions about when to tighten or relax prevention measures like masking and physical distancing. It also recommended that unvaccinated students and staff members keep wearing masks.

Chicago’s announcement came a day after the Virginia Department of Health and Department of Education said that masks should be worn indoors in public elementary schools, regardless of vaccination status. The state’s return-to-school guidelines also encourage mask-wearing indoors in middle and high schools for those who are unvaccinated. But Virginia stopped short of issuing a universal mask mandate, leaving that decision up to schools.

Chicago’s decision on masks was based in part on recommendations from the American Academy of Pediatrics, which took a more conservative approach than the C.D.C. by recommending that everyone over age 2 wear masks this fall, even if they have been vaccinated. But both the A.A.P. and the C.D.C. support a return to in-person learning.

With the new academic year just weeks away, mask rules are being closely watched — sometimes provoking controversy. On Thursday, two advocacy groups for parents in California filed a lawsuit against the state’s governor and top health officials over the state’s requirement that children must wear masks at school even if they are vaccinated.