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 Dutch epidemiological and contact survey data, to evaluate how alternative closure and reopening strategies for elementary and secondary schools might have shaped the course of the pandemic in the Netherlands from early 2020 through late 2021. Using counterfactual simulations combined with time-varying elasticity analysis, we found that school closures had the potential to mitigate transmission and reduce burden on healthcare. However, their effectiveness was highly context-dependent, influenced by school type, contact patterns, policy timing, and population immunity. Our findings indicate that the contributions of different age groups to transmission shifted over time. Adults were the primary drivers early in the pandemic, followed by adolescents in late 2020 and early 2021, and children in late 2021. These dynamics were reflected in the changing relative impact of different educational levels on overall transmission and hospital burden. Secondary schools had a greater effect on national hospitalizations in 2020, while elementary schools became more important in 2021, partly due to lower prior infection-induced immunity and negligible vaccination coverage among children compared to adolescents. Together, these results highlight the importance of accounting for age-specific transmission dynamics and changing epidemiological landscape when evaluating school closure and reopening strategies. Our study supports a more targeted and adaptive approach to school policies in future pandemics.