The First Test of Vaccination

[A] Edward Jenner's experiment of 1796 is usually described as the beginning of vaccination, and it was, but not because the underlying idea was new. Deliberate infection with smallpox itself, in the hope of producing a mild case and lifelong protection, had been practised in parts of Asia and Africa for generations and was known in London by the 1720s. [B] It worked, and it killed perhaps one patient in fifty.

[C] What Jenner tested was a piece of rural folklore: that milkmaids who caught cowpox, a trivial illness, never afterwards caught smallpox. He inoculated a boy with cowpox and, some weeks later, exposed him to smallpox. The boy did not fall ill. [D] The step that mattered was not the observation, which others had made, but the deliberate challenge afterwards, which turned a piece of country wisdom into a claim that could be tested and, in principle, refuted. Jenner's own explanation of why it worked was wrong in nearly every particular, and this did not matter at all. The procedure spread across Europe within a decade of his report, long before anyone could say what immunity was.