Differences in Polio Mortality by Socioeconomic Status in Two Southern Ontario Counties, 1900–1937

The collective polio literature of the mid-twentieth century developed a model centered on age at infection. In this model, known as the hygiene hypothesis, risk of severe polio increased with socioeconomic status (SES) because higher SES was associated with older age at infection. Rural residence was also linked to increased polio risk due to older age at infection. Crowding and larger family size were associated with earlier age at infection and thus reduced the risk of severe polio. In contrast, according to the intensive-exposure hypothesis proposed by Nielsen and colleagues (2001, 2002), exposure to the poliovirus within the home was linked to increased severity of infection, making larger family size and crowding important risk factors. Data for polio deaths in Wentworth and York counties, including the cities of Hamilton and Toronto, from 1900 to 1937 were gathered from a variety of archival sources and socioeconomic class was coded using the five-point composite score scale from Hauser (1982). The results provide support for the intensive-exposure hypothesis as an addition to the traditional polio model. Age at death increased with status score during the earlier 1900–1929 period, but not in the 1930–1937 period. The overall proportions of polio deaths in the various status scores were stable over both periods and disproportionately prevalent in status score three (skilled blue collar). This analysis of polio mortality provides a more nuanced picture of the disease and its relation to SES in a time of rapidly changing socioecological conditions.