Regional changes in temperature-related mortality before and during the COVID-19 pandemic: a continental modelling analysis in 805 European regions.

Background: The COVID-19 pandemic drastically disrupted usual seasonal mortality patterns, creating challenges in assessing temperature-related mortality. While previous studies explored the effect of temperature on SARS-CoV-2 transmission, few examined its relationship with mortality during the pandemic, often excluding COVID-19 deaths or relying on pre-pandemic models. In this study, we developed an innovative methodological framework that accounts for COVID-19 waves, allowing us to estimate changes in the short-term effects of temperature on mortality and assess the role of adaptation and maladaptation before and after the onset of the pandemic.

Methods: We analyzed pre- (2015-2019) and pandemic (2020-2023) mortality data from Eurostat, covering 805 contiguous regions across 32 European countries. To adjust for COVID-19 deaths, we selected specific time windows during COVID-19 waves, and increased the degrees of freedom (d.f.) for these windows as necessary until achieving well-behaved residuals.

Findings: Adjusting for COVID-19 deaths reduced uncertainty in the pandemic association, providing more precise estimates. When adjusting for COVID-19 deaths, we observed a significant reduction in cold and heat-related mortality risks in all sub-regions except in the southern regions for heat, which experienced a significant increase. When assessing the role of adaptation between pre- and pandemic periods, we observed significant changes for heat risks in southern and western regions with higher risks in the pandemic period than in the pre-pandemic one. For cold, all sub-regions except the southern ones had higher risks in the pre-pandemic period.

Interpretation: Our work defines a new innovative methodological framework for future epidemiological studies using data from the pandemic period. The proposed methodology demonstrates the importance of using pandemic data and adjusting for COVID-19 deaths to accurately capture current vulnerabilities. The findings highlight different regional adaptation processes and underscore the need for enhanced heat adaptation measures, particularly in vulnerable regions.