<P> A systematic review and meta analysis conducted in 2014 demonstrated that the BCG vaccine reduced infections by 19--27% and reduced progression to active TB by 71% . The studies included in this review were limited to those that used interferon gamma release assay . </P> <P> The duration of protection of BCG is not clearly known . In those studies showing a protective effect, the data are inconsistent . The MRC study showed protection waned to 59% after 15 years and to zero after 20 years; however, a study looking at Native Americans immunized in the 1930s found evidence of protection even 60 years after immunization, with only a slight waning in efficacy . </P> <P> BCG seems to have its greatest effect in preventing miliary TB or TB meningitis, so it is still extensively used even in countries where efficacy against pulmonary tuberculosis is negligible . </P> <P> A number of possible reasons for the variable efficacy of BCG in different countries have been proposed . None have been proven, some have been disproved, and none can explain the lack of efficacy in both low - TB burden countries (US) and high - TB burden countries (India). The reasons for variable efficacy have been discussed at length in a WHO document on BCG . </P>

How does the bcg vaccine make you immune to tb