Age-specific Excess Mortality Patterns During the 1918-1920 Influenza Pandemic in Madrid, Spain

Abstract

Although much progress has been made to uncover age-specific mortality patterns of the 1918 influenza pandemic in populations around the world, more studies in different populations are needed to make sense of the heterogeneous mortality impact of this deadly pandemic. Here we assess the absolute and relative magnitude of 3 pandemic waves in city of Madrid between 1918-1920 based on age-specific all-cause and respiratory excess death rates. We estimate excess death rates using a Serfling model with a parametric bootstrapping approach to calibrate baseline mortality levels with quantified uncertainty. We then estimate excess all-cause and pneumonia and influenza mortality rates for different pandemic waves and age groups. Age-specific analyses reveal the youngest and oldest experienced the highest excess mortality rates, and young adults faced the highest standardized mortality risk. Waves differed in strength; the peak standardized mortality risk occurred during the herald wave in spring 1918, but the highest excess rates occurred during the fall and winter of 1918-1919. We find little evidence to support a 'w'-shaped age-specific excess mortality curve. Our results indicate acquired immunity may have tempered a protracted fall wave, but recrudescent waves following the initial two outbreaks heightened the total pandemic mortality impact.