Excess mortality patterns during 1918-1921 influenza pandemic in the state of Arizona, USA

Abstract

Purpose: Our understanding of the temporal dynamics and age-specific mortality patterns of the 1918-1921 influenza pandemic remains scarce due to lack of detailed respiratory mortality datasets in the United States and abroad.

Methods: We manually retrieved individual death records from Arizona during 1915-1921 and applied time series models to estimate the age specific mortality burden of the 1918-1921 influenza pandemic. We estimated influenza-related excess mortality rates and mortality rate ratio increase over baseline based on pneumonia and influenza (P&I), respiratory, tuberculosis and all-cause death categories.

Results: Based on our analysis of 35,151 individual mortality records from Arizona, we identified three successive pandemic waves in spring 1918, fall 1918-winter 1919 and winter 1920. The pandemic associated excess mortality rates per 10,000 population in Arizona was estimated at 83 for P&I, 86 for respiratory causes, 84 for all-causes and 9 for tuberculosis. Age-specific P&I and tuberculosis excess death rates were highest among 25- to 44-year-olds and individuals ≥65 years, respectively. The 25- to 44-year-olds and 5- to 14-year-olds had highest P&I and tuberculosis mortality impact respectively when considering the ratio over background mortality.

Conclusions: The 1918-1921 influenza pandemic killed an estimated 0.8% of the Arizona population in three closely spaced consecutive waves. The mortality impact of the fall 1918 wave in Arizona lies in the upper range of previous estimates reported for other US settings and Europe, with a telltale age distribution of deaths concentrated among young adults. We identified a significant rise in tuberculosis-related mortality during the pandemic, lending support to the hypothesis that tuberculosis was a risk factor for severe pandemic infection. Our findings add to our current understanding of the mortality impact of this pandemic in the US and globally.

Keywords: 1918–1919; Age pattern; Arizona; Cause of death; Excess mortality; Influenza pandemic; Time-series; Tuberculosis.