Influence of vaccination-challenge interval on the protective efficacy of bacille Calmette-Guérin against low-virulence Mycobacterium tuberculosis.

The influence of vaccination-infection interval on protection induced by bacille Calmette-Guérin (BCG) was studied in an animal model of experimental airborne tuberculosis. Guinea pigs were simultaneously skin-tested with mammalian tuberculin and intracellularin and vaccinated with BCG-Copenhagen (strain no. 1331). At weekly intervals thereafter, groups of animals were infected by the respiratory route with about five viable units of a recently isolated strain of Mycobacterium tuberculosis of low virulence. The animals were necropsied six weeks after challenge, and tubercle bacilli recovered from primary lung lesions, primary lesion-free lung lobes, and spleens were counted. Protection was defined as a significant reduction in the number of bacilli recovered from the tissues of vaccinated as compared with unvaccinated animals. The data obtained for two of the three tissues indicated that BCG-Copenhagen induced a significant level of protection against this low-virulence of M. tuberculosis.