Influence of the virulence of Mycobacterium tuberculosis on protection induced by bacille Calmette-Guérin in guinea pigs.

The protective efficacy of two bacille Calmette-Guérin (BCG) vaccines was examined in guinea pigs infected by the respiratory route with strains of Mycobacterium tuberculosis differing in virulence. Virulence was defined as the degree of tissue damage (weight) of primary lesions excised from lungs of unvaccinated guinea pigs killed 28-42 days after infection. Groups of animals vaccinated with BCG-Copenhagen (strain no. 1331), a vaccine of high potency, or those vaccinated with BCG-Prague (strain no. 725), an experimental vaccine of low potency, and groups given placebo were challenged six weeks later with one of three challenge strains differing in virulence. Protection was assessed from the difference in the number of tubercle bacilli recovered from excised primary lung lesions or from primary lesion-free lung lobes of vaccinated vs. unvaccinated animals. The virulence of the challenge strain influenced the efficacy of BCG vaccination; however, the results of other studies with a laboratory strain were in general replicated.