Olivia Reyes, Elizabeth C Lee, Pratha Sah, Cécile Viboud, Siddharth Chandra, Shweta Bansal

The factors that drive spatial heterogeneity and diffusion of pandemic influenza remain debated. Here, we characterize the spatio-temporal mortality patterns of the 1918 influenza pandemic in British India and study the role of demographic factors, environmental variables, and mobility processes on the observed patterns of spread. We analyze fever and all-cause excess mortality across 206 districts in India during the period of January 1916 to December 1920, and control for variation in seasonality particular to India. Our analysis reveals that the 1918 autumn wave in India matches signature features of influenza pandemics with high disease burden among young adults, (moderate) spatial heterogeneity in burden, and highly synchronized outbreaks across the country deviating from annual seasonality. Importantly, we also find that population density and rainfall explain the spatial variation in excess mortality, and that long-distance travel via railroad is predictive of the observed spatial diffusion of disease. Our work integrates a spatio-temporal analysis of mortality patterns during the 1918 influenza pandemic in India with data on underlying factors and processes to reveal transmission mechanisms in a large, intensely connected setting with significant climatic variability. The characterization of such heterogeneity during historical pandemics is crucial to our ability to prepare for future pandemics.