Skip to main content

COVID-19 Cases and Transmission in 17 K–12 Schools — Wood County, Wisconsin, August 31–November 29, 2020

Our take —

Evidence from this prospective study on in-person K-12 schooling and COVID-19 transmission in 17 schools in Wisconsin suggests it is possible to keep schools open while also keeping school-borne SARS-CoV-2 transmissions low, even when local community transmission rates are very high. However, due to lack of routine surveillance testing for asymptomatic infections and difficulties determining the sources of cases detected within schools, the total number of school-related infections in this study was likely undercounted.

Study design

Prospective Cohort

Study population and setting

The goal of this study was to provide a comprehensive summary of SARS-CoV-2 transmission within Wisconsin schools kept open during the height of the state’s COVID-19 epidemic. This study took place in 17 K-12 schools in Wood County, Wisconsin between August 31 and November 29, 2020. In total, 8 elementary and 9 secondary schools participated, including both public and private school districts. There were 4,876 students and 654 staff members in the participating schools during the study period. In Wood County, the vast majority of students attended in-person school, with only 12.4% enrolled in virtual education. During the study period, participating schools reported a variety of COVID-19 mitigation measures, including the following: provision of free masks, mask requirements indoors and within 6 feet of anyone while outdoors, class cohorting, restricted seating arrangements during lunch hour and within classrooms, and home isolation and quarantine for students and staff with COVID-19 symptoms or for students with siblings in the school exhibiting COVID-19 symptoms.

Student mask compliance and attendance was reported by teachers through weekly online surveys. Cases of COVID-19 in schools were determined through public testing and reporting by school administrators. Determination of whether confirmed SARS-COV-2 infections among school students and staff had occurred within or outside of school was done collaboratively through investigations conducted by school and local public health authorities. County-level case counts and percent positivity among individuals testing for SARS-COV-2 were assessed using the county COVID-19 public dashboard and compared to case rates in schools. No surveillance testing (i.e., asymptomatic testing) was conducted among either the school or local county population. During the study period, local transmission of COVID-19 was high with 7-40% of county SARS-COV-2 tests being positive.

Summary of Main Findings

There were 191 confirmed cases of SARS-CoV-2 identified among students and staff members in the 17 participating schools. Among these cases, only 3.1% (n=7) were attributed to within school transmission events as determined by school and public health authorities. During the study period, the SARS-COV-2 case rate in the local community was 5,466 per 100,000 persons, whereas it was only 3,453 per 100,000 in the participating schools (i.e., a 37% relative reduction in cases among the school versus county population). Overall, student mask compliance during the study period was high, with weekly compliance ranging from 92.1% to 97.4%; however, the teacher response rate to mask compliance surveys was only 54%

Study Strengths

The study linked a variety of data sources to provide a comprehensive summary of SARS-CoV-2 transmission within Wisconsin schools kept open during the height of the state’s COVID-19 epidemic. Individual case investigations enabled researchers to determine whether COVID-19 cases detected in schools likely arose from school or community exposure.


There were several major limitations to this study. First, because no routine surveillance testing was performed, we only have data on individuals who opted to be tested, most likely because they were symptomatic. As a result, we do not know the true number of infections in either the schools or local community. It is likely that both the testing behaviors and rates of cases that were symptomatic were different among the two populations. If, for example, school aged children are more likely to be asymptomatic, they will test less often per infection, and overall have more undetected cases. Furthermore, the two populations are likely to be different in social patterns, health status, age, etc. that makes comparing the school and general community populations difficult. Second, attributing cases to particular infection locations is very difficult in practice. Notably, all 191 cases detected in the school population here would have had exposure to both school and household/community exposures. Given that no details were provided on how cases were determined to have arisen from school vs. community settings, it is impossible to assess whether methods of source attribution were potentially biased. Third, mask wearing behavior was only reported by half of all teachers, and these reports may have been biased toward reporting higher mask compliance as a consequence of social desirability-biases. Lastly, it is unclear whether the schools selected for participation in this study represented all schools in the county. This raises questions about how schools were selected for the study and how many of the community cases identified were actually attributable to school vs. community transmission.

Value added

This study adds to a growing body of literature on SARS-CoV-2 transmission within schools. This is one of the first studies to systematically assess SARS-CoV-2 transmission within schools in the United States during a period of high community transmission.

This review was posted on: 12 March 2021