Does bacille Calmette-Guérin scar size have implications for protection against tuberculosis or leprosy?

Abstract

Setting: Total population study in Karonga District, northern Malawi, in which the overall vaccine efficacy of bacille Calmette-Guérin (BCG) has been found to be −7% against tuberculosis and 54% against leprosy. Objective: To examine the relationship between BCG scar size and protection against tuberculosis and leprosy. Design: Cohort study in which 85 134 individuals were screened for tuberculosis and 82 265 for leprosy between 1979 and 1984, and followed up between 1986 and 1989. Results: Of the BCG scar positive individuals whose scars were measured, 31 3 2471 were later identified with tuberculosis and 81 31 879 with leprosy. In 19 114 individuals, of whom 17 developed tuberculosis, tuberculin induration was measured at first examination. Mean scar sizes increased with increasing tuberculin induration in all except the oldest individuals. Mean scar sizes were lowest in individuals aged < 10 years, highest in individuals aged 10–29 years and intermediate in older individuals. There was some evidence ( P = 0.08) for an increase in tuberculosis risk with increasing scar size, which probably reflects the known correlation between scar size and tuberculin status at the time of vaccination. There was no clear association between BCG scar size and leprosy incidence. Conclusions: We find no evidence that increased BCG scar size is a correlate of vaccine-induced protective immunity against either tuberculosis or leprosy.