SARS-CoV-2 transmission in secondary schools and the influence of indoor environmental conditions

We quantified pre-vaccination (2020/2021) SARS-CoV2 transmission in secondary education and investigated the association with indoor climate and COVID19 mitigation measures. To this end, we combined environmental and virological measurements in experimental settings and field studies in 18 secondary schools, supplemented with 4 detailed SARS-CoV2 outbreak studies. During a period of low population immunity and limited measures in education, there appeared to be undetected SARS-CoV2 transmission in schools. Furthermore, national government measures aimed at reducing contacts proved effective in reducing the SARS-CoV2 incidence in schools, but an additional positive effect of self-initiated local measures by schools was not found. Adjustments in ventilation practices were made during the study, which somewhat limited the generalizability of the conditions. However, an effect of higher CO2 concentration in classrooms on SARS-CoV2 incidence in schools was not found and there was no evidence for large-scale environmental contamination with SARS-CoV2, even during outbreaks. The combined results suggest a limited role for airborne transmission over longer distances under the conditions prevailing in schools during the study. The research has also contributed to optimization of sampling techniques for virus-containing aerosols and CO2 measurements in classrooms.