Preparation of Inactivated St. Louis Encephalitis Virus Vaccine from Hamster Kidney Cell Culture. ∗

Abstract

Summary

The large plaque variant of the P-15 strain of SLE virus which grows to a titer of 10⁸ to 10⁹ TCID₅₀/0.1 ml in HKC was inactivated by formalin and tested for antigenicity. The virus was harvested in medium 199 containing 2% H.AL, and was readily and apparently predictably inactivated by 1:4000 formalin at 37°C. Loss by nitration prior to activation was in the expected range of 0.3 log. The inactivated preparation when tested for the minimal immunogenic dose (MID) in mice gave a low value, indicative of excellent immunogenicity. The MID for a preparation with a preinactivation titer of 10^-7-3/0.1 ml was 0.0079 ml, whereas that with a titer of 10^-8-7/0.1 ml furnished an MID of <0.0015 ml. The latter vaccine elicited an unexpectedly good response in guinea pigs in an antigen extinction test. The protection the mice developed from such vaccine exceeded that of an earlier mouse brain St. Louis virus vaccine and that of a recently developed HKC JBE vaccine.