In addition to the sensitization with antigens from environmental mycobacteria that cause usually small cross-reactions, BCG vaccination also induces cross-reactions.

Shortly after vaccination, the distribution of tuberculin skin test reactions is virtually indistinguishable from that caused by infection with M. tuberculosis (Chingleput at 2 1/2 months).  Over time, this distribution moves to the left as the immunologic response wanes.  However, this waning appears to differ, depending on age at vaccination and perhaps also on the type of vaccine strain.

BCG vaccination is the most obvious source of non-specific tuberculin reactions.  In tuberculin skin test surveys, BCG vaccinated persons have thus usually been excluded from the analysis of the prevalence of tuberculous infection.  As BCG coverage is commonly in excess of 80% in countries routinely using it, the exclusion of the majority is likely to bias the findings. 

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