The BCG story: lessons from the past and implications for the future.

BCG (bacille Calmette-Guérin) vaccines are at once among the least satisfactory and yet the most widely used of all vaccines today. Their variable efficacy against tuberculosis and leprosy is still not understood and points to a fundamental unsolved problem in vaccine immunology. The extensive use of BCG vaccines means that there are few BCG-free populations in the world that would be suitable for trials of future antimycobacterial vaccines. These facts have implications with regard to strategies for the development and testing of new vaccines against mycobacterial diseases.