Where BCG vaccination is used its influence on the interpretation of the tuberculin skin test can be strong, depending on the age vaccination is given, the time elapsed since vaccination, and the BCG strain. In this study from Korea (South), BCG is shown to give a distribution that closely resembles that of infection with M tuberculosis after about two months following vaccination. This is not surprising as BCG is an attenuated strain of M bovis which is antigenically closely related to M tuberculosis. With this strain of BCG and vaccination given to school children, reactivity waned quickly and after one year the distribution of reaction sizes had shifted to the left, but a considerable proportion still showed relative large reactions. After five years, reactivity had further waned and only small proportion had still reaction sizes exceeding 10 millimeters of induration. In other settings, the influence of BCG on tuberculin skin test reactivity might be of much longer time.
