Risk, efficacy, and attenuation in debates over smallpox vaccination in Montreal 1870-1876

In Montreal, in the early 1870s, physicians complained of a high failure rate for public and private smallpox vaccination. The supply of good stock vaccine had become increasingly problematic for physicians at a time when smallpox was endemic. Renewed interest in the nature and efficacy of vaccine-induced immunity against smallpox was spurred on by problems with the vaccine and by popular resistance to the technology. This paper focuses on the complex and recursive relationship between theories of viral attenuation, vaccine-induced immunity, and the assessment of the probable efficacy of vaccination in the field. Debates over vaccine's efficacy from the early 1870s quickly polarized physicians into camps of pro and anti-vaccinationists. However, proponents on both sides of the debate used concepts of attenuation to describe the varied clinical presentation of smallpox and to account for the inconsistent success of vaccination. Pro-vaccinationists tended to argue that the apparent failure of vaccination was a result of the variability of the vaccine quality, but conceived of smallpox as a relatively fixed species. By 'fixing the contagion', the effectiveness of the vaccine during a particular epidemic could be measured through standard hospital data and the technique of vaccination improved accordingly. Anti-vaccinationists maintained that it was the contagion itself that was mutable and that the protective effects ascribed to vaccination were simply a manifestation of a natural process of attenuation. Anti-vaccinationists were particularly successful in using concepts of attenuation to provide an alternative interpretation of data used to support vaccine's efficacy. In both cases, it was the fundamental belief of disbelief in the principle of vaccination that shaped the reading of nineteenth century data on smallpox and vaccination.