Animals have been used in the industrialized production of human vaccines since vaccine farms were established to harvest cowpox virus from calves in the late 1800s. From that point, and through the first half of the 20th Century, most vaccines would continue to be developed with the use of animals, either by growing pathogens in live animals or by using animal cells.

Although many vaccines and anti-toxin products were successfully developed this way, using animals in vaccine development – particularly live animals – is not ideal. Research animals are costly and require extensive monitoring, both to maintain their health and to ensure the continued viability of the research. They may be carrying other bacteria or viruses that could contaminate the eventual vaccine, as with polio vaccines from the mid 20th century that were made with monkey cells and eventually found to contain a monkey virus called SV40, or Simian Virus 40. (Fortunately, the virus was not found to be harmful to humans.) Moreover, some pathogens, such as the chickenpox virus, simply do not grow well in animal cells. //

For these and other reasons, using cell culture techniques to produce vaccine viruses in human cell strains is a significant advance in vaccine development.