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I read (from Nature Volume507, S4–S7 (06 March 2014) :

For reasons that are poorly understood, BCG protects only infants; it is ineffective in older children and adults. Its efficacy also depends on latitude. BCG works better, and protects people at later ages, farther from the equator. Both effects may result from interfering exposure to noninfectious mycobacteria that are closely related to both BCG and TB. These bacteria are more abundant closer to the equator, and while infants are unlikely to have encountered them, children and adults have more exposure over time.

I am unable to understand the relation between exposure to noninfectious mycobacteria and lessening of efficacy of BCG vacccine. What exactly is the "interference" caused by the noninfectious mycobacteria ?

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The exposure to environentally occurring Mycobacteria causes a immune response to these (mostly unspecific, since no real infections occur). It seems that immunizations with BCG in these individuals is causing either no, or only a very attenuated immune response This is shown in this ("BCG-induced increase in interferon-gamma response to mycobacterial antigens and efficacy of BCG vaccination in Malawi and the UK: two randomised controlled studies.") in which individuals with a high environmental exposure to mycobycteria showed only a very small response to this vaccination. If this natural level of immunity is enough to produce immunity to tuberculosis is unknown (and can not be tested for obvious reasons).

Another possibility is that the natural occurring immunity blocks the vaccine strains so they get eliminated before they induce an immune response. See here. This is called the masking theory,if you want to read further, have a look at this three reviews, they give a good overview over the field:

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