In May 1796, Jenner took matter from a cowpox sore on milkmaid Sarah Nelmes and inoculated 8-year-old James Phipps with it.
Then the step no review board would allow today: after Phipps recovered, Jenner deliberately exposed him to smallpox. The disease never took.
That was the whole experiment — one child, one challenge, no controlled trial. It backed the old milkmaid observation that cowpox protects against smallpox. Why it worked, Jenner could not say.