Reconstructing age-specific social contact patterns across 28 EU/EEA countries during the COVID-19 pandemic: linking non-pharmaceutical interventions to social mixing, 2020-2022

In response to the COVID-19 pandemic, most EU/EEA countries sequentially introduced non-pharmaceutical interventions (NPIs) to reduce social contacts and mitigate transmission. The CoMix study captured real-time changes in social mixing, but substantial spatiotemporal gaps remain, with some countries lacking pandemic contact data. We developed a model-based framework to estimate time-varying changes in age-specific contact patterns and relate these changes to NPIs recorded in the ECDC-JRC Response Measures Database. This relationship was used to predict contacts during periods without CoMix observations. For countries without CoMix data, we reconstructed temporal age-specific mixing patterns using estimates from countries with pandemic contact data selected through a country-matching process based on socio-demographic characteristics and similarity in NPI implementation. We generated age-specific contact matrices for 28 EU/EEA countries spanning 2020-2022. Reconstructed patterns captured substantial temporal and cross-country variation in social mixing and its evolving association with NPIs across diverse European contexts. Our framework provides estimates that extend incomplete pandemic contact data across both time and countries, yielding a consistent set of age-specific contact matrices for the EU/EEA. These estimates address important gaps in empirical social-contact data and provide valuable insights into the interplay of NPIs and social mixing patterns which is of critical importance to improving epidemic modelling and policy making.