Wikipedia:

At the age of 13, Jenner was apprenticed to Dr. Ludlow in Sodbury. He observed that people who caught cowpox while working with cattle were known not to catch smallpox. He assumed a causal connection. The idea was not taken up by Dr. Ludlow at that time. After Jenner returned from medical school in London, a smallpox epidemic struck his home town of Berkeley, England. When he advised the local cattle workers to be inoculated, the farmers told him that cowpox prevented smallpox. This confirmed his childhood suspicion, and he studied cowpox further, presenting a paper on it to his local medical society.

Saying "He ran an experiment and it worked" hides the initial correlational observation that let him to try that experiment.