Vaccination against Human Leprosy Bacillus Infections of Mice: Protection by BCG Given during the Incubation Period

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Abstract
Summary: The experimental infection of mice with Mycobacterium leprae and the natural infection of humans have uniquely long incubation periods. To see whether vaccination given during the incubation period of the experimental infection would provide protection, groups of mice were given one or two injections of BCG vaccine at intervals before or after they were challenged with M. leprae. The amount of protection was assessed by microscopic counts of M. leprae in the tissues 7 and 10 months after challenge, at a time when the leprosy infection had developed in the controls. It was found that vaccine given 1 or 2 months before challenge afforded the amount of protection observed previously with this schedule. Vaccines given just after challenge provided no protection. Vaccines given progressively later during the incubation period (closer to the logarithmic phase of growth of M. leprae) caused increasing protection, up to the same level as that provided by vaccine given 1 or 2 months before challenge. These changes in the amount of protection according to the schedule of the vaccine suggest that the vaccines exert their effect on M. leprae shortly after challenge or during its logarithmic phase, and that vaccine protection in the mouse usually deteriorates in a few months. Evidence from the literature on vaccination against M. tuberculosis indicates that protection afforded to man would last very much longer, so that BCG vaccine given early or late in the incubation period of leprosy in humans would be expected to provide protection.

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