Why Does Influenza Hit the Poor More Than the Rich? A 1931 Social Epidemiology Article That Broke New Ground in the History of Confounding, Mediation, and Interaction

Abstract

In 1931, Edgar Sydenstricker, the former statistician of the US Public Health Service, challenged the common belief that the 1918 influenza outbreak had affected "the rich and the poor alike." Using data from 112,317 participants in a 1918 US national survey, he observed that, on the contrary, both morbidity and mortality from the flu had been higher among the poor than among the rich. To explain these differences, Sydenstricker stratified the analyses by 2 measures of affluence collected in the survey: "economic status" (from "very poor" to "well-to-do") and household crowding (i.e., number of people per household room). Economic status was associated with influenza attack rates within categories of crowding, but not the opposite, suggesting that characteristics of poverty other than "household congestion" were the culprit of the poor's higher influenza burden. Attack rate ratios for influenza in infants and older adults were greater for the poor or very poor. Sydenstricker reanalyzed an already 12-year-old data set in the context of the Great Depression to build the evidence base relating poverty to ill health. For this purpose he used a stratification approach to assess confounding, mediation, and interaction before the concepts were formally named.

Keywords: confounding; effect modification; history; interaction; mediation.