[jes5199]

wikipedia: “From 1796 to the 1880s, the vaccine was transmitted from one person to another through arm-to-arm vaccination.”

(Variolation used real smallpox, arm-to-arm used the vaccine virus instead) https://en.wikipedia.org/wiki/Smallpox_vaccine#Types

“As medical researchers subjected viruses to serial passage, inadequate recordkeeping resulted in the creation of laboratory strains with unclear origins.[95]: 4 By the late 19th century, it was unknown whether the vaccine originated from cowpox, horsepox, or an attenuated strain of smallpox.”

https://en.wikipedia.org/wiki/Smallpox_vaccine#Origin