Variable Efficacy
The most controversial aspect of BCG is the variable efficacy found in different clinical trials that appears to depend on geography. Trials conducted in the UK have consistently shown a protective effect of 60 to 80%, but those conducted elsewhere have shown no protective effect, and efficacy appears to fall the closer one gets to the equator.
The first large-scale trial evaluating the efficacy of BCG was conducted from 1956 to 1963, and involved 54,239 school children who received BCG at the age of 14 or 15; this study showed an efficacy of 84% up to five years after immunization. However, a US Public Health Service trial of BCG in Georgia and Alabama published in 1966 showed an efficacy of only 14%, and did much to convince the US it did not want to implement mass immunization with BCG. A further trial conducted in South India and published in 1979 (the "Chingleput trial"), showed no protective effect.
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.
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.
Read more about this topic: Bacillus Calmette–Guérin
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