Plague Immunization, II. Relation of Adverse Clinical Reactions to Multiple Immunizations with Killed Vaccine

During a 21-year period, 1,219 persons received a total of 18,751 inoculations of killed plague vaccines. Each vaccinee was closely observed for adverse clinical reactions to vaccination. Twenty-nine percent demonstrated some local reaction, and 20% showed some systemic reactions. Severe reactions were infrequent. Although reactions to inoculation of plague vaccines appeared to reflect individual idiosyncrasy, rates of reactions could be modified by adjusting the dose. Reduction of vaccine dose for booster inoculations from 1.0–2.0 × 109 bacilli/ml to 5 × 108 Yersinia pestis/ml resulted in a 70% reduction in local reactions and a 65% reduction in systemic reactions. The data suggest that administration of killed vaccine according to current public health regulations should result in minimal reaction rates. However, severe reactions to plague vaccine, although infrequent, do occur and are cause for concern.