Kits found in collections relating to the American Civil War correspond to a time of known medical crisis and intervention to prevent smallpox outbreaks. Vaccination ‘kits’ and their biological contents (scabs, lymph) provide evidence of early vaccination methods and materials and remain in medical collections/archives across the globe. However, both the origin of VACV and its natural host or reservoir are also unknown. In 1939, it was recognised that the smallpox vaccine strains being used in the twentieth century were distinct from CPXV and these VACV strains had become the predominant smallpox vaccines. However, ‘cowpox’ and ‘horsepox’ are likely misnomers, for neither cows nor horses are considered the natural reservoirs of these viruses, and the absence of endemic CPXV or HSPV outside of Europe suggests geographically restricted hosts. Both are thought to produce comparatively self-limiting infections in humans with negligible mortality rates. On the basis of Edward Jenner’s work, cowpox virus (CPXV) was assumed to have been involved in historical vaccination, although horsepox virus (HSPV) and ‘equination’ are also cited.
The lack of standardisation in vaccination practices and propagation throughout most of its history means that historical vaccine strains may be any one of several OPXVs. The World Health Organization’s success in eradicating smallpox using vaccinia virus (VACV) (1980) was in part due to the broad protective immunity induced by infection with one OPXV against subsequent infection by another. Despite these profound public health benefits, the origins and diversity of the viruses used in the early vaccination programs remain uncertain. Smallpox remains the only human infectious disease eradicated, a global accomplishment achieved through widespread coordinated vaccination.
Smallpox epidemics were caused by variola virus (VARV), a human-specific member of the Orthopoxvirus (OPXV) genus of the Poxviridae, and resulted in high mortality and morbidity with survivors frequently disabled or disfigured.
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