Initial clinical evaluation of a new rocky mountain spotted fever vaccine of tissue culture origin

Currently available Rocky Mountain spotted fever (RMSF) vaccines are relatively ineffective in preventing infections in humans and contain considerable amounts of contaminating egg protein. A new formalin-inactivated vaccine was prepared by sucrose density gradient centrifugation of the Sheila Smith strain of Rickettsia rickettsii grown in chick embryo cell tissue culture. The new product has greater protective immunogenicity in rhesus monkeys and guinea pigs than commercial vaccines. Six volunteers without immunologic evidence of prior exposure to RMSF received from one to three inoculations of the vaccine diluted 1:10, and there were two benign local reactions. Titers of antibody (determined by microagglutination and indirect fluorescence techniques) increased in all recipients as did lymphocyte transformation responses to specific rickettsial antigen. Ten volunteers were immunized twice with vaccine diluted 1:3; there were no local reactions, and immunologic responses were similar to those in the six volunteers in the first group. The proper dosage and immunization schedule for the vaccine must be determined in further studies.