Transmission of SARS-CoV-2 in Norwegian schools: A population-wide register-based cohort study on characteristics of the index case and secondary attack rates

Rotevatn, T. A.; Larsen, V. B.; Johansen, T. B.; Astrup, E.; Suren, P.; Greve-Isdahl, M.; Telle, K. E.

Objectives: To assess transmission of SARS-CoV-2 in schools mainly kept open during the COVID-19 pandemic. Design: Population-wide register-based cohort study. Setting: Primary and lower secondary schools in Norway have been open during the academic year 2020/2021 with strict infection prevention and control (IPC) measures in place. All identified contacts including student and staff members were urged to get tested following a positive SARS-CoV-2 case in a school. Participants: All students and educational staff in Norwegian primary and lower secondary schools from August 2020 to June 2021. Main outcome measures: Overall secondary attack rate (SAR14) was operationalized as the number of secondary cases (among students and/or staff) in the school by 14 days after the index case, divided by the number of students and staff members in the school. Moreover, we calculated SAR14-to-students, denoting transmission from all index cases to students only, SAR14-to-school staff, denoting transmission from all index cases to staff members only. We also calculated these measures in stratified samples consisting of student index cases or school staff index cases. Results: From August 2020 to June 2021 there were 4,078 index cases, 79% were students and 21% were school staff. In the majority (55%) of schools with an index case, no secondary cases were observed by 14 days, and in 16% of the schools there were only one secondary case within 14 days. Overall SAR14 was found to be 0.33% (95%CI 0.32-0.33). Staff-to-staff transmission (SAR14 0.45%, 95%CI 0.40-0.52) was found to be slightly more common than student-to-student (SAR14 0.33%, 95%CI 0.32-0.34) and student-to-staff (SAR14 0.28%, 95%CI 0.25-0.30) transmission. Conclusions: Our results confirm that schools have not been an important arena of transmission of SARS-CoV-2 in Norway and therefore support that schools can be kept open with IPC measures in place.