Prophylactic Effectiveness of Live and Killed Tularemia Vaccines

Abstract

Summary: An attenuated USSR vaccine culture of P. tularensis was examined for dissociation and found heterogeneous with regard to colony type morphology. Live BV, the more immunogenic of two variants isolated from the culture, proved effective for the protection of the mouse against s.c. challenge with strain SCHU S4; in contrast, killed cultures were not immunogenic. Guinea pigs vaccinated with live BV cells and challenged with strain SCHU S4 exhibited an appreciably longer survival time than animals vaccinated with killed preparations but eventually they succumbed. Although SCHU S1-11, one of several colony type variants of reduced virulence isolated from strain SCHU S1, proved equally as immunogenic for the mouse as BV and afforded the guinea pig greater protection than BV, the relatively high residual virulence of this variant for the guinea pig precluded its use as a vaccine strain. Serial animal passage of BV permitted the isolation of a culture that was ultimately selected for the production of live vaccine. The culture was designated the live vaccine strain (LVS) and is currently being used for the production of lyophilized live vaccine for use in man. Strain SCHU S4 proved superior to strain 503 as a challenge strain for quantitating the protection afforded test animals administered live vaccines. Data on the immunogenicity and storage stability of lyophilized live tularemia vaccine produced from cells harvested from GCHA and more recently from MCPH proved sufficiently encouraging to have warranted an extension of this study to a systematic evaluation of the vaccine in the monkey and subsequently in man.