On May 14, 1796, the first documented vaccination against smallpox, by Edward Jenner, an English physician and scientist, took place. Jenner inoculated eight-year-old James Phipps with matter from a cowpox sore on the hand of milkmaid Sarah Nelmes. Phipps suffered a local reaction and felt poorly for several days but made a full recovery. In July 1796, Jenner inoculated Phipps with matter taken from a fresh human smallpox sore, as if he were variolating the boy, in an attempt to challenge the protection from cowpox. Phipps remained healthy. Jenner next demonstrated that cowpox matter transferred in a human chain, from one person to the next, provided protection from smallpox.
Jenner was not precisely sure about the nature of the cowpox material he
used. He suspected that cowpox actually came from horsepox; in other words, he
speculated that cows became infected with the same agent that caused a similar
disease in horses. Recent genetic analysis of old samples of smallpox vaccine
have revealed that the samples were more closely related to horsepox virus than
cowpox virus.
