The 1918 influenza pandemic stands out because of the unusual age pattern of high mortality. In the United States, another feature merits scientific scrutiny: against a historical backdrop of extreme racial health inequality, the pandemic produced strikingly small ratios of nonwhite to white influenza and pneumonia mortality. We provide the most complete account to date of these racial disparities in 1918, showing that, across U.S. cities, they were almost uniformly small. We examine four potential explanations for this unexpected result, including  socio-demographic factors like segregation,  city-level implementation of non-pharmaceutical interventions (NPIs),  exposure to the milder spring 1918 “herald wave,” and  early-life exposures to other influenza strains resulting in differential immunological vulnerability to the 1918 flu. While we find little evidence for 1-3, we offer suggestive evidence that racial variation in early-life exposure to the 1889-1892 influenza pandemic shrunk racial disparities during the 1918 pandemic. We also raise the possibility that differential behavioral responses to the herald wave may have protected nonwhite urban populations. By providing a comprehensive description and careful examination of the potential drivers of racial inequality in mortality during the 1918 pandemic, our study provides a framework to consider interactions between the natural history of particular microbial agents and the social histories of the populations they infect.
Awarded the 2023 Evolution, Biology and Society Section’s Best Article Award