Importance: Different mitigation measures are mandated in schools worldwide to control the spread of SARS-CoV-2. The efficacy of most measures, however, has not been investigated thus far. Objective: To investigate the usefulness of FFP-2 masks in classrooms to prevent the spread of SARS-CoV-2. Design: A retrospective comparative cohort study of infection rates (evaluated by PCR screening in school) in students wearing FFP-2 masks continuously and students in sports classes with limited face mask use. Setting: A single-center evaluation comparing classes (middle school: age 10-16 years, 4-year high school: age 14-20 years) with a high sports focus (SF), with regular classes during the Delta and Omicron waves (September 2021-April 2022). Participants: In total, 616 children/families were invited to participate in the comparative evaluation, and 614 (99.7%) followed this invitation by providing relevant information concerning their SARS-CoV-2 infection status. A total of 213 legal guardians (for children < 14 years) and 401 adolescents (14 years and older) reported SARS-CoV-2 infections during the 2021/22 school year. Main Outcomes and Measures: A comparative analysis of cumulative SARS-CoV-2 infection rates in sports and non-sports classes (the 7-day classroom incidence of SARS-CoV-2 infections, and potential secondary infections among school classmates). Results: Cumulative SARS-CoV-2 infection rates were clearly higher in sports classes (with limited mask use) than in non-sports classes (continuous mask use). After the relaxation of the mitigation measures, students in non-sports classes, however, showed a clear catch-up of infections, leading to a higher incidence of infections during this phase. By the end of the observation period (April 30, 2022), only a small difference in cumulative SARS-CoV-2 infection rates (p=0.037, phi=0.09) was detected between classes with a sports focus and those

without a sports focus. Conclusions and Relevance: Wearing FFP2 face masks reduces the risk of SARS-CoV-2 infection if strict mitigation measures are applied. Following the relaxation of strict measures, previously protected students show a significant catch-up infection rate. Thus, continuous face mask use postpones rather than avoids SARS-CoV-2 infection in many cases. Therefore, the advantage of reduced transmission must be carefully balanced against the disadvantages associated with mask wearing throughout schools.

Competing Interest Statement
The authors have declared no competing interest.

https://www.medrxiv.org/content/10.1101/2022.07.24.22277968v2