When jenner developed the first vaccine he was using the observation of milk maids who]

Who became immune to smallpox after getting infested by cowpox.

Explanation:

A successful vaccine for smallpox was developed by Edward Jenner in 1796. He used the observation of milkmaids who became immune to smallpox after getting infected by the cowpox virus. It is due to the fact that the virus, which leads to cowpox infection is identical to the smallpox virus. When the milkmaids got infected by the cowpox virus, their immune cells generated antibodies, which can also destruct virus for smallpox, and therefore, they were not affected by the smallpox virus.