Jenner's smallpox vaccination (cowpox inoculation)

Before 1796, the only protection against smallpox was variolation: deliberate inoculation with material taken from a smallpox pustule. The procedure usually gave immunity, but 1 to 2 percent of those variolated died, and the mild smallpox it caused could spread and start epidemics. Edward Jenner, a country physician in Berkeley, Gloucestershire, had spent years gathering reports of dairymaids who seemed to resist smallpox after catching the milder cowpox. The observation was not new among English farmers: in 1774 Benjamin Jesty, a farmer at Yetminster, had deliberately given cowpox to his wife and two sons during a smallpox outbreak. No one had tested the idea systematically.

On May 14, 1796, Jenner inoculated eight-year-old James Phipps with material taken from a cowpox pustule on the hand of milkmaid Sarah Nelmes. Six weeks later, he inoculated the boy repeatedly with smallpox material. Phipps did not get smallpox. Jenner sent an account to the Royal Society in 1797. The society returned it as preliminary, and he had to wait until cowpox broke out again in March 1798 to inoculate more children. His 1798 monograph, "An Inquiry into the Causes and Effects of the Variolae Vaccinae," described 23 cases, including 15 people who had caught cowpox years earlier and did not develop smallpox when later exposed to it.

Jenner paid a printer to publish the Inquiry, and some London physicians doubted it. His sample was small, no one knew how cowpox protected, and in early trials at London's smallpox hospital many vaccinated patients broke out in smallpox-like pustules, probably because the cowpox material was contaminated with smallpox virus. Uptake was still fast. By 1801 vaccination had reached France, Spain, the United States and Canada, and Napoleon ordered his army vaccinated in 1805.

Jenner's method used a related, milder virus to protect against a deadly one. The word "vaccine" comes from vacca, Latin for cow, after Jenner's original material. In 1807 Bavaria became the first state to require vaccination of army recruits. In 1967 the World Health Organization launched its intensified smallpox eradication program. The last natural case was in Somalia in 1977, and in 1980 the WHO declared smallpox eradicated, the only human infectious disease eliminated from nature.