An Inactivated Eastern Equine Encephalomyelitis Vaccine Propagated in Chick-Embryo Cell Culture: I. Production and Testing*,†

Abstract A formalin-inactivated EEE vaccine prepared in chick-embryo cell cultures was evaluated in man. Studies with 16 volunteers given three doses of 0.5 ml subcutaneously showed only mild reactions in some persons; no meaningful changes in clinical laboratory values occurred during the evaluation period. The serologic responses of volunteers suggested that a two-dose series with a 28-day interval between doses would result in significant and persistent titers. This schedule was employed for immunization of 92 “at-risk” laboratory workers with no known experience with EEE. Twenty-eight days after the two-dose series, 93% had a log10 neutralization index (LNI) of ≥1.7, with all but two persons having an LNI of ≥2.0. Use of the vaccine in 117 persons who had been inoculated with EEE vaccine demonstrated the immunogenicity of the vaccine when given intradermally in a 0.1-ml dose. Of 95 persons with a titer of <1.7 at the time of the booster dose, 85 (89%) responded to a level of ≥1.7 by 28 days after the booster. This is indicative of the high immunogenicity of the vaccine given intradermally in a 0.1-ml dose.