Measuring the efficacy of vaccination in affording protection against plague.

The relationship of F1 antibody titre to protection against plague was investigated by subjecting seropositive laboratory rats to virulent challenge and observing for survival. The passive haemagglutination (PHA) test in microtitre was employed for serology. Rats vaccinated with live vaccine EV76 (51f), killed U.S.P. vaccine, or F1 antigen and challenged by subcutaneous inoculation of 1 x 10(3) to 5 x 10(5)Yersinia pestis survived at similar rates that, overall, equalled 6% at titres less than 1:16, 46% at titres of 1:32-1:64, 90% at titres of 1:128-1:256, and 96% at titres of 1:512-1:1024. Rats vaccinated with F1 antigen and rats that had been infected previously were challenged intranasally with 8.9 x 10(4)Y. pestis and subsequently demonstrated similar rates of survival that was zero at titres less than 1:128, 86% at titres of 1:128-1:256, and 100% at titres of 1:512-1:1024. The significance of titre of F1 antibody as a measure of seroimmunity against acute bubonic or pneumonic plague is discussed for rats, monkeys, and man.