Cowpox vaccination was used to protect against smallpox in the late 1700s.

“well the oldest older oldest trick is is a a live non-human virus i mean the cow pox was was the first vaccine in the late 1700s which so basically the thinking there was that that not that it was thinking at all since this was the late 1700s before we actually knew what viruses were i mean it was just phenomenology but um that that now we know that uh the cow pox virus is energetically related energetically related close enough to human smallpox that immunization with one can protect you against disease caused by the other”