First-generation left|thumb|274x274px|Ovine smallpox vaccine, manufactured by growing live vaccinia in sheep, 1980s The first-generation vaccines are manufactured by growing live vaccinia virus in the skin of live animals. Most first-generation vaccines are calf lymph vaccines that were grown on the skin of cows, but other animals were also used, including sheep. The development of freeze-dried vaccine in the 1950s made it possible to preserve vaccinia virus for long periods of time without refrigeration, leading to the availability of freeze-dried vaccines such as Dryvax.
First-generation left|thumb|274x274px|Ovine smallpox vaccine, manufactured by growing live vaccinia in sheep, 1980s The first-generation vaccines are manufactured by growing live vaccinia virus in the skin of live animals. Most first-generation vaccines are calf lymph vaccines that were grown on the skin of cows, but other animals were also used, including sheep The development of freeze-dried vaccine in the 1950s made it possible to preserve vaccinia virus for long periods of time without refrigeration, leading to the availability of freeze-dried vaccines such as Dryvax.