Counterfactual evaluation of elementary and secondary school policies in the COVID-19 pandemic

Understanding the role of elementary and secondary schools in SARS-CoV-2 transmission is essential for designing effective public health policies. We developed a stochastic age-stratified transmission model, fitted to epidemiological and contact survey data, to evaluate how alternative school closure and reopening strategies might have shaped the course of the pandemic in the Netherlands from early 2020 through late 2021. Using counterfactual simulations and time-varying elasticity analysis, we found that school closures could mitigate transmission and reduce healthcare burden. However, their effectiveness was highly context-dependent, influenced by school type, contact patterns, policy timing, and population immunity. Age groups driving transmission shifted over time, with adults dominating early in the pandemic, adolescents in late 2020 and early 2021, and children in late 2021. Accordingly, secondary schools had a greater impact on national hospitalizations in 2020, while elementary schools became more important in 2021, partly due to lower infection-induced and negligible vaccine-induced immunity among children compared to adolescents. Together, these results underscore the importance of accounting for age-specific transmission dynamics and the evolving epidemiological landscape when evaluating school closure and reopening strategies. Our study supports a more targeted and adaptive approach to school policies in future pandemics.