Background: There has been considerable debate around mask requirements in schools in the United States and other countries during the Covid-19 pandemic. To date, there have been no randomized controlled trials of mask requirements in children. All analyses of the effectiveness of school mask mandates have relied on observational studies. The Centers for Disease Control in the U.S. have released multiple observational studies suggesting that school mask mandates significantly reduce case rates. However, there have also been numerous additional US and international observational studies finding no significant effect of school mask mandates on pediatric cases.
Methods: Our study replicates a highly cited CDC study showing a negative association between school mask mandates and pediatric SARS-CoV-2 cases. We then extend the study using a larger sample of districts and a longer time interval, employing almost six times as much data as the original study. We examine the relationship between mask mandates and per-capita pediatric cases, using multiple regression to control for differences across school districts.
Findings: Replicating the CDC study shows similar results; however, incorporating a larger sample and longer period showed no significant relationship between mask mandates and case rates. These results persisted when using regression methods to control for differences across districts. Interpretation: School districts that choose to mandate masks are likely to be systematically different from those that do not in multiple, often unobserved, ways. We failed to establish a relationship between school masking and pediatric cases using the same methods but a larger, more nationally diverse population over a longer interval. Our study demonstrates that observational studies of interventions with small to moderate effect sizes are prone to bias caused by selection and omitted variables. Randomized studies can more reliably inform public health policy.