Background K-12 schools play a crucial role not only as educational settings but also as hubs for social interaction among children, making them potential drivers of disease transmission within families and the broader community. Little is known about the impact of mandatory school masking policies on community SARS-CoV-2 infection rates during the COVID-19 pandemic. This study addressed this gap by evaluating the association between mandatory school masking policies and community infection rates, accounting for temporal and regional variations.

Methods We conducted a retrospective quasi-experimental study using the synthetic control method. The study focused on the fall 2021 school reopening period in the United States, a time when most U.S. schools returned to fully in-person learning but accompanied by substantial variation in masking policies. Analyses controlled community characteristics prior to reopening and baseline infection rates.

Findings Counties with mandatory school masking policies experienced lower SARS-CoV-2 infection rates compared to counties with non-mandatory policies. During the first nine weeks after school reopening, there was a reduction of 1,096 cases per 100,000 people (95% confidence interval: 880 to 1,310 fewer cases per 100,000 people). This association was influenced by baseline infection rates, population density, and mobility patterns.

Interpretation Mandatory school masking policies were associated with notable reductions in community SARS-CoV-2 infection rates during the fall 2021 school reopening period. This study highlights the potential role of school masking as a public health intervention in mitigating community transmission.

Funding Centers for Disease Control and Prevention and the National Institutes of Health

The authors have declared no competing interest.

The work was supported by the Centers for Disease Control and Prevention (CDC) and National Institutes of Health (NIH) under cooperative agreement U01CK000674 and grant number R01HD099348. The funders had no role in the study design, data collection, data analysis, data interpretation, or writing of the report. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of CDC or NIH.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This study was reviewed by the Institutional Review Board of the Children's Hospital of Philadelphia and was determined to be exempt from review as it involved analysis of publicly available, de-identified, aggregate data and did not constitute human subjects research under applicable regulations.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

Data for this study are available upon reasonable request to the corresponding author.