Secondary Transmission of COVID-19 in K-12 Schools: Findings From 2 States

We evaluated the impact of distancing practices on secondary transmission of severe acute respiratory syndrome coronavirus 2 and the degree of sports-associated secondary transmission across a large diverse cohort of schools during spring 2021.

Participating districts in North Carolina and Wisconsin and North Carolina charter schools offering in-person instruction between March 15, 2021 and June 25, 2021 reported on distancing policies, community- and school-acquired infections, quarantines, and infections associated with school-sponsored sports. We calculated the ratio of school-acquired to community-acquired infection, secondary attack rates, and the proportion of secondary transmission events associated with sports. To estimate the effect of distancing and bus practices on student secondary transmission, we used a quasi-Poisson regression model with the number of primary student cases as the denominator.

During the study period, 1 102 039 students and staff attended in-person instruction in 100 North Carolina school districts, 13 Wisconsin school districts, and 14 North Carolina charter schools. Students and staff had 7865 primary infections, 386 secondary infections, and 48 313 quarantines. For every 20 community-acquired infections, there was 1 within-school transmission event. Secondary transmissions associated with school sports composed 46% of secondary transmission events in middle and high schools. Relaxed distancing practices (<3 ft, 3 ft) and increased children per bus seat were not associated with increased relative risk of secondary transmission.

With universal masking, in-person education was associated with low rates of secondary transmission, even with less stringent distancing and bus practices. Given the rates of sports-associated secondary transmission, additional mitigation may be warranted.