Retrospective evaluation of school-related measures on pre-vaccination transmission dynamics of SARS-CoV-2

Schools are important settings for respiratory virus transmission and a major focus of pandemic control measures in the absence of effective pharmaceutical interventions. During the COVID-19 pandemic, decisions regarding school closures and openings were highly debated due to uncertainties about their population-level impact. Using an age-stratified transmission model formally fitted to hospitalization and seroprevalence data in a Bayesian framework, we retrospectively assessed the effect of school closures and openings on SARS-CoV-2 transmission in Portugal before mass vaccination. We compared the observed epidemic trajectory with two counterfactual scenarios: one in which schools remained open during the first lockdown of spring 2020 and another in which they closed for in-person education following the summer holidays in autumn 2020. We found that keeping schools open during the first lockdown could have led to a worse situation, with hospitalizations rising far beyond observed levels in spring 2020. However, closing schools after the summer holidays alone would not have been sufficient to prevent the autumn wave of hospitalizations in 2020, though it could have mitigated its impact. These findings underscore that the epidemiological impact of school closures is highly context-dependent, influenced by mitigation measures within schools and the intensity of transmission outside them. While closures can support control efforts, especially early into a pandemic, their benefits must be weighed against substantial societal costs. Our results suggest that integrated strategies combining school-based and broader community interventions are essential to effectively reduce transmission during future pandemics.