Western equine encephalomyelitis vaccine produced in chick embryo cell cultures.

A method was developed for production of a freeze-dried Western equine encephalomyelitis vaccine from virus propagated in chick embryo cell culture monolayers maintained with a serum-free medium. A sufficient concentration of virus accumulated in the cell culture fluids prior to the occurrence of viral cytopathology to permit the production of a vaccine relatively free from serum and cellular proteins. Inoculation with two mouse ld(50) doses of virus per 100 tissue culture cells was found to yield reproducible high virus titers at a convenient harvest time. These harvests were inactivated at 22 C by 0.05% formalin within 48 hr. Potency test results, as measured by the protection of immunized guinea pigs against an intracerebral virus challenge, indicated that the vaccine produced from the virus propagated in cell culture was equal in potency to a lot of whole chick embryo vaccine used to immunize laboratory and field workers subject to a high risk of infection.