Assessing the impact of SARS-CoV-2 prevention measures in schools by means of agent-based simulations calibrated to cluster tracing data

How to safely maintain schools open during a pandemic is still controversial. We aim to identify those measures that effectively control the spread of SARS-CoV-2 in schools. By control we mean that each source case infects less than one other person on average. Here, we analyze Austrian data on 616 clusters involving 2,822 student-cases and 676 teacher-cases with the aim to calibrate an agent-based epidemiological model in terms of cluster size and transmission risk depending on age and clinical presentation. With this model, we quantify the impact of preventive measures such as room ventilation, reduction of class size, wearing of masks during lessons, and school entry testing by SARS-CoV2-antigen tests. We find that 40% of all clusters involved no more than two cases, and 3% of the clusters only had more than 20 cases. The younger the students, the more likely we found asymptomatic cases and teachers as the source case of the in-school transmissions. Different school types require different combinations of measures to achieve control of the infection spreading: In primary schools, it is necessary to combine at least two of the afore-mentioned measures. In secondary schools, where contact networks of students and teachers become increasingly large and dense, a combination of three measures is needed. A sensitivity analysis indicated that the cluster size might increase up to three-fold in secondary schools for virus variants with an increased transmissibility by 50%, and that poorly executed or enforced mitigation measures might increase the cluster size by a factor of more than 30. Our results suggest that school-type-specific combinations of measures, when strictly adhered to, allow for a controlled opening of schools even under sustained community transmission of SARS-CoV-2. However, large clusters might still occur on an infrequent, however, regular basis. It is shown explicitly that strict adherence to the measures is a necessary condition for successful control.