Protective effect of BCG against leprosy and its subtypes: a case-control study in southern Vietnam

A case-control study was conducted to assess the protective effect of intradermal BCG against leprosy and its subtypes in southern Vietnam. A total of 177 cases were selected with a distribution by subtypes as follows: 38 TT, 23 BT, 51 BB, 36 BL, 22 LL, and 7 indeterminate. Two controls were matched with a case for age, sex, ethnic group, socioeconomic status, and district area. The odds ratio assessing the protective effect of BCG varied from 0.44 (0.19-1.03) in the BB subtype to 3.00 (0.24-37.5) in indeterminate leprosy; whereas its overall value was 0.71 (0.45-1.10) for leprosy per se. When all borderline leprosy types were pooled, the protective effect of BCG was found significant with an odds ratio of 0.48 (0.27-0.84). In the polar forms of leprosy, TT and LL, the odds ratio was > 1 with large confidence intervals. It is possible that BCG induces a shift in the immune response to a higher level of cell-mediated immunity. When BCG vaccination is given after primary infection with Mycobacterium leprae, this shift could be the cause of an increase in the risk of the occurrence of milder and transient forms of the disease. In TT forms BCG might reinforce the preexisting subclinical immunopathological reactions, and in stable LL forms BCG might be unable to induce any protective form of immunity. These results confirm the important variability in the protection offered by BCG with respect to the different types of leprosy, and may have important implications for the design and the interpretation of vaccine trials that should take into account the respective proportions of leprosy forms observed in the study region.