COVID-19’s unfortunate events in schools: mitigating classroom clusters in the context of variable transmission

Abstract

Widespread school closures occurred during the COVID-19 pandemic. Because closures are costly and damaging, many jurisdictions have since reopened schools with control measures in place. Early evidence indicated that schools were low risk and children were unlikely to be very infectious, but it is becoming clear that children and youth can acquire and transmit COVID-19 in school settings and that transmission clusters and outbreaks can be large. We describe the contrasting literature on school transmission, and argue that the apparent discrepancy can be reconciled by heterogeneity, or ``overdispersion'' in transmission, with many exposures yielding little to no risk of onward transmission, but some unfortunate exposures causing sizeable onward transmission. In addition, respiratory viral loads are as high in children and youth as in adults, pre- and asymptomatic transmission occur, and the possibility of aerosol transmission has been established. We use a stochastic individual-based model to find the implications of these combined observations for cluster sizes and control measures. We consider both individual and environment/activity contributions to the transmission rate, as both are known to contribute to variability in transmission. We find that even small heterogeneities in these contributions result in highly variable transmission cluster sizes in the classroom setting, with clusters ranging from 1 to 20 individuals in a class of 25. None of the mitigation protocols we modeled, initiated by a positive test in a symptomatic individual, are able to prevent large transmission clusters unless the transmission rate is low (in which case large clusters do not occur in any case). Among the measures we modeled, only rapid universal monitoring (for example by regular, onsite, pooled testing) accomplished this prevention. We suggest approaches and the rationale for mitigating these ``unfortunate events'', even if they are expected to be rare.

Clustering and longitudinal change in SARS-CoV-2 seroprevalence in school children in the canton of Zurich, Switzerland: prospective cohort study of 55 schools

“…Finally, a stochastic modelling study of infection spread in schools has shown that some, although minimal, clustering of infections is likely even when major prevention and screening strategies are implemented. 32 …”

“…Finally, a stochastic modelling study of infection spread in schools has shown that some, although minimal, clustering of infections is likely even when major prevention and screening strategies are implemented. 32 In contrast to these retrospective and modelling studies, our study offers a prospective population level view, which corresponds to school structure because of sampling at school and class levels. Additionally, having measured the baseline seroprevalence in June-July 2020, we were able to study the number of children who were newly seropositive and their clustering in classes in the autumn.…”

Clustering and longitudinal change in SARS-CoV-2 seroprevalence in school-children: prospective cohort study of 55 schools in Switzerland

“…Finally, a few ecological studies tried to estimate the overall effect of closing and opening schools on the development of the pandemic (in terms of diagnosed and reported cases and deaths) [29– 31], with major limitations of uncontrolled confounding, high level of aggregation (e.g., pooling school and university closures as one intervention, or analyzing aggregated outcomes on country-level) and potentially measuring the outcome in a population not exposed to the intervention. Finally, a stochastic modelling study of infection spread in schools have shown that some, although minimal, clustering of infections (outbreaks) is likely to happen even if major prevention and screening strategies are implemented [32].…”

Comment empêcher le SRAS-CoV-2 d’entrer dans les écoles

“…Puisqu'une grande proportion des enfants infectés par le SRAS -CoV-2 ne développent aucun symptôme, les stratégies de dépistage de symptômes ne peuvent vraisemblablement pas empêcher tous les enfants infectieux d'entrer dans les écoles. Par conséquent, les mesures de santé et de sécurité autres que le dépistage -notamment la distanciation physique, l'hygiène des mains, le port du masque, l'amélioration de la ventilation et les activités d'apprentissage à l'extérieur -qui y sont mises en place sont essentielles pour y prévenir la propagation de l'infection [6][7][8] . Il importe de recueillir des données sur la transmission secondaire à ces endroits pour évaluer ces mesures et les adapter selon les besoins.…”