The BCG Strain Pool: Diversity Matters

The bacillus Calmette-Guérin (BCG) is of central importance to the vaccination programs of many countries with a high incidence of tuberculosis (TB).1 However, despite its well-recognized efficacy against miliary and meningeal TB in young children, vaccination with BCG confers only limited and/or variable protection against pulmonary TB in adolescents and adults, with an efficacy ranging from 0 to 80% according to several clinical trials.1 The overall interpretation of BCG vaccine efficacy and the resulting recommendations are further complicated by the fact that BCG is not a single, pharmacologically well-defined vaccine but, rather, a pool of different BCG daughter strains (Figure 1) that have acquired phenotypic and genotypic variations during decades of in vitro culturing in different laboratories.