Bayesian ensemble modelling to monitor excess deaths during summer 2022 in Switzerland

Abstract: Switzerland experienced one of the warmest summers during 2022. Extreme heat has been linked to increased mortality. Monitoring the mortality burden attributable to extreme heat is crucial to inform policies, such as heat warnings, and prevent heat-related deaths. In this study, we evaluate excess mortality during summer 2022, identify vulnerable populations and estimate temperature thresholds for heat warnings. We use nationwide mortality and population data in Switzerland during 2011-2022 by age, sex, day and canton. We develop a Bayesian ensemble modelling approach with dynamic population to predict expected mortality in summer 2022 and calculate excess by comparing expected with observed mortality. We account for covariates associated with mortality such as ambient temperature, national holidays and spatiotemporal random effects to improve predictions. After accounting for the effect of the COVID-19 pandemic, we found a 3% (95% credible interval: 0%-6%) excess mortality during summer 2022. We observed a total of 456 (5-891) excess deaths during summer 2022 in people older than 80 years. There was weak evidence of excess mortality in the other age groups. The highest excess mortality was observed in July (12%: 4%-19%), the hottest month in 2022. We also found that for heatwaves longer than four days, the minimum excess mortality temperature threshold in the oldest age group is the 70th percentile of the temperature. To reduce future summer excess mortality in Switzerland, we propose targeted heat warnings to older populations and reducing the temperature threshold when weather forecasts predict periods of extreme heat of four days or longer.