Scientists have not determined exactly when the change or mutation occurred, but the effects of vaccinia and cowpox virus as vaccine are nearly the same.
When a smallpox epidemic occurred he advised the local cattle workers to be inoculated, but they told him that their previous cowpox infection would prevent smallpox.
Two children were vaccinated immediately before departure, and when cowpox pustules had appeared on their arms, material from these lesions was used to vaccinate two more children.