In countries with high rates of TB, BCG is often given to infants at the time of birth because it helps prevent the more serious forms of TB disease from developing in children. In some countries BCG is given to the same person several times during childhood and early adult life, in an effort to maintain an immunity to TB. Repeated vaccination increases the likelihood of causing a positive skin test but may not increase protection against TB.

Unfortunately, the positive effect of BCG in protecting infants and young children from endemic areas from the lethal forms of TB does not extend to the adult years. Thus, many people develop active tuberculosis even though they received BCG, even in multiple doses, in earlier years. Since BCG has been used so widely and for such a long time, if it were effective it is unlikely that one third of the world's population would now have TB infection and that two million people a year worldwide would die of TB.

BCG can cause a positive skin test, especially if the vaccine was given after early infancy, if it was given several times, and if it was given within the last five to ten years. Nevertheless, since many persons who have immigrated to the U.S. are at risk of TB even though they received BCG, the recommendation in the United States is to interpret skin tests and recommend treatment regardless of whether you have had BCG. In other words, if you have a positive skin test and are from a part of the world where TB is common, you should assume that it is due to TB exposure and implies a risk of future disease and not that it's positive because of the BCG. Nevertheless, you should let your doctor know if you had a BCG, when and if you had it more than once.

Yes. BCG continues to be studied intensively around the world for ways in which its effectiveness can be increased. Research toward a more effective TB vaccine has also been initiated. However, a new TB vaccine should not be expected for at least another 20 years.