Effect of BCG on the risk of leprosy in an endemic area: a case control study.

The effect of BCG on the risk of leprosy was measured using a case-control design in an area endemic for the disease. In this study, 397 newly diagnosed cases and 669 controls matched for age, sex and locality were selected from a defined population. Information on exposure to BCG, contact with another case of leprosy, and relevant socioeconomic variables were obtained from the subjects. Having infectious (multibacillary) and noninfectious (paucibacillary) contacts in the household increased the risk of disease 11.7 times (p less than 0.001) and 2.7 times (p less than 0.001), respectively. Overall, the protection offered by BCG was not significant (odds ratio = 0.8; p = 0.17). However, BCG appeared to increase the risk for indeterminate leprosy (adjusted odds ratio = 2.7; p = 0.09) while protecting against borderline disease (adjusted odds ratio = 0.39; p = 0.03). It is possible that BCG causes a shift in the overall cell-mediated immune response, thus increasing the risk for milder and transient forms of leprosy while protecting against more serious forms. These findings may have important implications for the design and interpretation of vaccine trials. Namely, trials should be designed to measure the protective efficacy of vaccines against the more serious forms of leprosy, which have the greatest public health significance.