Social network-based cohorting to reduce the spread of SARS-CoV-2 in secondary schools: A simulation study in classrooms of four European countries

Abstract

Background: Operating schools safely under pandemic conditions is a widespread policy goal. We analyse the effectiveness of classroom cohorting, i.e., the decomposition of classrooms into smaller isolated units, in inhibiting the spread of SARS-CoV-2 in European secondary schools and compare different cohorting strategies.

Methods: Using real-world network data on 12,291 adolescents collected in classrooms in England, Germany, the Netherlands, and Sweden in 2010/2011, we apply agent-based simulations to compare the effect of forming cohorts randomly to network-based cohorting. Network-based cohorting attempts to allocate out-of-school contacts to the same cohort to prevent cross-cohort infection more effectively. We consider explicitly minimizing out-of-school cross-cohort contacts, approximating this information-heavy optimization strategy by chained nominations of contacts, and dividing classrooms by gender. We also compare the effect of instructing cohorts in-person every second week to daily but separate in-person instruction of both cohorts.

Findings: We find that cohorting reduces the spread of SARS-CoV-2 in classrooms. Relative to random cohorting, network-based strategies further reduce infections and quarantines when transmission dynamics are strong. In particular, network-based cohorting inhibits superspreading in classrooms. Cohorting that explicitly minimizes cross-cohort contacts is most effective, but approximation based on chained nominations and classroom division by gender also outperform random cohorting. Every-second-week instruction in-person contains outbreaks more effectively than daily in-person instruction of both cohorts.

Interpretation: Cohorting of school classes can curb SARS-CoV-2 outbreaks in the school context. Factoring in out-of-school contacts can achieve a more effective separation of cohorts. Network-based cohorting reduces the risk of outbreaks in schools and can prevent superspreading events.

Funding: None.

Fig. 1

Cross-cohort out-of-school ties for different cohorting strategies in an example classroom from the CILS4EU data. Nodes represent students and ties among nodes represent out-of-school contacts with classmates. Colors indicate the cohort to which students have been allocated. Cohorts have the same size.

Fig. 2

Simulation model for transmission of SARS-CoV-2 within classrooms.

Fig. 3

Average proportion of infected students in case of no cohorting and two types of random cohorting. Proportions and 95% confidence intervals. Results across entire parameter space are in Fig. S1.

Fig. 4

Total number of ties between classmates in the out-of-school contact network and (average) number of cross-cohort ties for different cohorting strategies across all classrooms.

Fig. 5

Epidemiological outcomes of different cohorting strategies: Proportion of outbreaks spreading to the second cohort, proportion of students infected, and excess proportion of students quarantined. Proportions and 95% confidence intervals. Numbers above excess proportion quarantined indicate proportion to be added to obtain total proportion quarantined (+ 1/2 of proportion clinical). Results across entire parameter space are in Fig. S2.

Fig. 6

Distribution of infections in the 5% and 1% largest outbreaks for different cohorting strategies.