Plague Immunization. VI. Vaccination with the Fraction I Antigen of Yersinia pestis

Abstract

The vaccination of man against plague with Fraction I (FI), the specific capsular antigen of Yersinia pestis, stimulates significant mouse-protective indices (MPI) in approximately two-thirds of those inoculated subcutaneously, and the immunity persists for three months. Revaccination with FI raises the antibody levels of nearly 95% of previously immunized individuals to an extent rarely encountered in studies of plague vaccine. The MPI recorded are roughly equivalent to those observed in the sera of patients convalescing from plague. Although FI may cause delayed allergic reactions, the evidence is conclusive that it is an antigen of major importance among the “dead” Y. pestis vaccines currently in use.