Age-specific COVID-19 case dynamics during school closure periods: Evidence from 12 european countries

School closures were among the most widely adopted non-pharmaceutical interventions against COVID-19, yet evidence on their association with infection dynamics remains uncertain given their substantial economic and educational costs. This observational study describes age-specific COVID-19 case dynamics during school closure periods across 12 European countries using Generalised Additive Models with zero-crossing derivative analysis and Transfer Entropy. Case dynamics were heterogeneous and non-monotonic across age groups: case counts declined early but rose again within the window among the youngest cohorts (0-4 and 5-9 years). By contrast, the 10-14 group rose steadily with no turning point, and older adolescents and young adults (15-19 and 20-24 years) rose early and then declined. Transfer Entropy analysis, with Benjamini-Hochberg correction, revealed asymmetric directional relationship between age cohorts that varied across national contexts. These patterns indicate that case trajectories during closure periods were not uniform across youth age groups, with no age group showing a sustained, uninterrupted decline. Interpreted cautiously, they raise questions about the cost-benefit balance of blanket closure policies when weighed against their documented economic, educational, and distributional costs, and motivate more targeted, age-sensitive quasi-experimental evaluation for future pandemic preparedness.