How the 1918-1920 Influenza Pandemic Spread Across Switzerland - Spatial Patterns and Determinants of Incidence and Mortality

This study shows that the quality of the morbidity data is sufficient to allow for meaningful analyses of spatiotemporal dynamics and provides a significant contribution to understanding the 1918-1920 influenza pandemic in Switzerland by complementing existing mortality- focused research with a morbidity perspective. Previous studies have examined the spatial patterns of mortality during the 1918-1920 influenza pandemic and associated explanatory factors. However, while mortality reflects the severity of a pandemic, a full understanding requires analysis of both morbidity and mortality. For the first time, this study systematically analysed district-level morbidity data for all of Switzerland and their associations with several ecological determinants. The spatial pattern of morbidity with the spatial pattern of mortality were also compared to investigate potential differences. Spatial clustering was assessed using the Getis-Ord Gi* statistic, and geographically weighted regression was employed to evaluate local relationships between incidence and ecological variables. Across all waves, higher incidence rates were positively associated with population density, GDP per capita, the share of industry, and the number of private physicians per km{superscript 2}. Conversely, GDP per capita and industrial activity were associated with lower mortality, while a higher proportion of men in a district correlated with lower incidence and higher mortality. The share of individuals aged 20- 39 years was associated with both higher incidence and higher mortality. These findings highlight that those factors shaping morbidity patterns can differ from those influencing mortality, emphasizing the importance of examining both dimensions for a comprehensive understanding of pandemic dynamics.