BCG (Bacillus of Calmette and Guerin) is a vaccine currently used in many parts of the world against certain forms of TB. PPD reactivity caused by BCG generally wanes with the passage of time. Typically no reaction persists after 10 years. No reliable method exists today to distinguish TB reactions caused by BCG from those caused by an actual TB infection. Therefore, a positive PPD reaction in a BCG-vaccinated person is assumed to indicate actual infection with Mycobacterium Tuberculosis. The American Thoracic Society, as well as the CDC, states that a prior BCG vaccination should not affect the decision to treat latent TB infections. The same criteria should be used whether or not BCG was ever given. Preventive treatment for Latent TB Infections should be considered for any person who has a PPD skin reaction greater than or equal to 10mm.
Why do some countries use BCG?
BCG is sometimes recommended for the following:
Prevention of some infectious forms of TB, especially in children (i.e. TB meningitis, miliary TB).
Reduce the likelihood of TB infection in children exposed to high-risk environments for TB.
Considered for health care workers who are employed in settings in which the likelihood of infection by multi-drug resistant TB is high, in spite of implementing all safety precautions.
BCG is rarely used in the United States today.
Reference: April 26, 1996 MMWR Vol. 45 / No. RR-4