Transmission of SARS-CoV-2 in Norwegian schools during academic year 2020-21: population wide, register based cohort study

OBJECTIVE: To assess the risk of transmission of SARS-CoV-2 in schools in Norway mainly kept open during the covid-19 pandemic in the academic year 2020-21. DESIGN: Population wide, register based cohort study. SETTING: Primary and lower secondary schools in Norway open during the academic year 2020-21, with strict infection prevention and control measures in place, such as organisation of students into smaller cohorts. Contact tracing, quarantine, and isolation were also implemented, and testing of students and staff identified as close contacts. PARTICIPANTS: All students and educational staff in primary and lower secondary schools in Norway, from August 2020 to June 2021. MAIN OUTCOME MEASURES: Overall attack rate of SARS-CoV-2 transmission (AR14) was defined as the number of individuals (among students, staff, or both) in the school with covid-19, detected within 14 days of the index case, divided by the number of students and staff members in the school. AR14 to students (attack rates from all index cases to students only) and AR14 to school staff (attack rates from all index cases to staff members only) were also calculated. These measures for student and school staff index cases were also calculated separately to explore variation in AR14 based on the characteristics of the index case. RESULTS: From August 2020 to June 2021, 4078 index cases were identified; 3220 (79%) students and 858 (21%) school staff. In most (2230 (55%)) schools with an index case, no subsequent individuals with covid-19 were found within 14 days; in 631 (16%) schools, only one more individual with covid-19 within 14 days was found. Overall, AR14 was 0.33% (95% confidence interval 0.32% to 0.33%). When restricting index cases and subsequent individuals with covid-19 to students born in the same year, AR14 to students (0.56-0.78%) was slightly higher. CONCLUSIONS: Regarding the number of people infected with SARS-CoV-2 among students and staff, these results suggest that schools were not an important setting for transmission of the virus in Norway during the covid-19 pandemic in the academic year 2020-21.