Proactive vs. reactive COVID-19 screening in schools: lessons from experimental protocols in France during the Delta and Omicron waves

The COVID-19 pandemic prompted diverse policies to manage safety in schools, balancing infection control with educational continuity. This study assessed the impact of an experimental weekly screening protocol compared to nationally implemented reactive strategies in 25 primary schools in the Auvergne-Rhone-Alpes region of France during the Delta (November-December 2021) and Omicron (January-February 2022) waves. We used an agent-based model for SARS-CoV-2 transmission in schools parameterized with empirical data characterizing school contact over time to estimate the contribution of school transmission on overall cases and evaluate the effectiveness of weekly screening in reducing within-school infections. We parametrized the model to reproduce the Delta and Omicron variants dominant in the study period, accounting for introductions from community surveillance data. We fitted the model to the observed prevalence in 18 schools selected for the analysis. School transmission was estimated to account for 67% (IQR 53-78) of student cases in Rhone and 67% (IQR 50-82) in Savoie during the Delta wave, and 52% (IQR 47-57) in Rhone during the Omicron wave. The experimental weekly screening protocol was estimated to reduce transmission in school by 40% (IQR 18 - 53%) during the Delta wave and by 37% (IQR 30-45) during the Omicron wave, compared to the reactive strategies applied in the same period in the rest of the country. Adherence rates exceeding 80% during the study were critical to the protocols success, contributing to an earlier and sustained decline in prevalence. Weekly screening proved a more structured and effective approach to controlling transmission, supporting its inclusion in future pandemic preparedness plans to ensure safer learning environments. This study underscores the importance of proactive interventions to address asymptomatic spread in schools, emphasizing their role in pandemic response strategies.