Countries using BCG are usually those where high tuberculosis infection is common.
BCG is not always given at birth and some areas, India for example, do not have the resources to skin test before vaccination in most cases. Tuberculosis infection, therefore, may occur before vaccination.
Reported protection from BCG is 0 to 80 percent. Tuberculosis may occur after vaccination.
Not only is BCG not proven to be protective, it does not uniformly result in a positive PPD due to vaccine and host variability. Again, many countries lack the resources to retest after vaccination in order to revaccinate non-converters.
Skin test reactivity to a good BCG vaccination is generally less than to true tuberculosis infection, and it wanes faster over the years. Reactivity in adults due to BCG given in infancy is often less than 10 mm.
For all these reasons, it is reasonable to consider an adult with a significant PPD, especially a large reaction, to be infected with tuberculosis and act accordingly, regardless of the BCG history.
Young children who were recently vaccinated are more likely to be positive due to BCG, but the potential consequence of being wrong is greater and the potential benefit of INH prophylaxis is lifelong. Unless there was a known negative PPD before vaccination and a significant reaction 6 to 8 weeks after vaccination, children should be considered candidates for INH prophylaxis.
American Thoracic Society/CDC Official Statement:
The Tuberculin Skin Test, 1981
(Am. Rev. Resp. Dis. 1981:124, No. 3).