Rocky Mountain spotted fever: a comparative study of the active immunity induced by inactivated and viable pathogenic Rickettsia rickettsii

An experimental model of Rocky Mountain spotted fever (RMSF) was developed in man for quantitation of protective immunity induced by inactivated and viable pathogenic Rickettsia rickettsii. Attack rates were increased, and the incubation period shortened, by increasing the infecting dose of virulent rickettsiae. Administration of inactivated vaccines made from infected tick tissue or embryonated hens' eggs led to a rise in humoral antibodies in a low percentage of men. When given three to six months before exposure to virulent rickettsiae, both vaccines altered the response in volunteers by prolonging the incubation period and decreasing the frequency of clinical relapse. Six men who were four to 17 months convalescent from induced RMSF failed to develop clinical illness after challenge with 1 ID94 (94% infectious dose) of virulent rickettsiae. These studies indicate that solid immunity is imparted by RMSF illness, while inactivated vaccines offer minimal protection.