Disparities in influenza mortality and transmission related to sociodemographic factors within Chicago in the pandemic of 1918

Abstract

Social factors have been shown to create differential burden of influenza across different geographic areas. We explored the relationship between potential aggregate-level social determinants and mortality during the 1918 influenza pandemic in Chicago using a historical dataset of 7,971 influenza and pneumonia deaths. Census tract-level social factors, including rates of illiteracy, homeownership, population, and unemployment, were assessed as predictors of pandemic mortality in Chicago. Poisson models fit with generalized estimating equations (GEEs) were used to estimate the association between social factors and the risk of influenza and pneumonia mortality. The Poisson model showed that influenza and pneumonia mortality increased, on average, by 32.2% for every 10% increase in illiteracy rate adjusted for population density, homeownership, unemployment, and age. We also found a significant association between transmissibility and population density, illiteracy, and unemployment but not homeownership. Lastly, analysis of the point locations of reported influenza and pneumonia deaths revealed fine-scale spatiotemporal clustering. This study shows that living in census tracts with higher illiteracy rates increased the risk of influenza and pneumonia mortality during the 1918 influenza pandemic in Chicago. Our observation that disparities in structural determinants of neighborhood-level health lead to disparities in influenza incidence in this pandemic suggests that disparities and their determinants should remain targets of research and control in future pandemics.

Keywords: 1918; influenza; mortality; sociodemographic; transmission.

Fig. 1.

Cumulative incidence of influenza and pneumonia mortality per 1,000 by census tracts during the 7 wk of the epidemic in Chicago in 1918. The three empty regions represent the three census tracts for which there were no demographic data that were excluded from analysis. The southernmost empty region is a lake.

Fig. 2.

Point locations of influenza and pneumonia mortality occurring in each week from September 29 to November 16 superimposed on a choropleth map showing illiteracy rates by census tracts in Chicago in 1918.

Fig. S1.

Pandemic mortality and all-cause mortality from 1912 to 1917 by ward in four cities. Pandemic mortality, the sum of influenza and pneumonia deaths from September 28 to November 17, 1918 (Chicago), all-cause mortality in September, October, and November of 1918 (Baltimore), or annual 1918 influenza mortality (Buffalo and Philadelphia), is strongly linearly related to all-cause mortality. Blue lines show the best fit linear regression.

Fig. S2.

Spring and fall wave simulations: simulation 1 (Table S6). Stochastic realizations of the transmission model for selected census tracts in the (A) spring and (B) fall outbreaks. (C) Imposed positive relationship between transmissibility and illiteracy during both outbreaks. Fall attack rates in each census tract are highly correlated with (D) spring attack rates and (E) illiteracy rate.

Fig. 3.

Effective reproduction estimates by week for (A) illiteracy and (B) population density quartiles. Lines represent mean R at each week for 124 census tracts in each quartile, and shaded regions represent 95% bootstrapped CIs.

Fig. 4.

Short-term spatial dependence of influenza and pneumonia deaths occurring within the same week, ϕ(d₁, d₂). Spatial window d₂ − d₁ was held constant at 100 m. Estimates are plotted for the midpoint of each spatial window. The shaded regions represents 95% bootstrapped CIs.