Post-vaccination tuberculin sensitivity for assessing BCG vaccination in areas with high prevalence of non-specific sensitivity.

The relationship of non-specific sensitivity to post-vaccination tuberculin sensitivity has been the subject of controversy. This relationship has an important bearing on the common practice of using post-vaccination tuberculin sensitivity for assessing BCG vaccination in areas with a high prevalence of non-specific sensitivity. If post-vaccination tuberculin sensitivity is found to be dependent on the amount of already existing, naturally acquired, non-specific sensitivity it could not be considered a reliable measure of the success of BCG vaccination. In a recent BCG trial in India a large number of persons were tested with PPD-S and PPD-B and also vaccinated with BCG vaccine or injected with placebo, by random allocation. Mutually exclusive random samples of the study population were retested with PPD-S at 2 1/2 months, 2 1/2 years and 4 years after vaccination. At each post-vaccination test the tuberculin sensitivity obtained among the placebo-injected controls provided a measure of the tuberculin sensitivity due to non-specific sensitivity that would have been present in the absence of BCG vaccination. The difference between the mean size of reactions to the post-vaccination test among the vaccinated and among the unvaccinated controls provided a measure of tuberculin sensitivity that was wholly due to BCG vaccination. Thus it was possible to separate and study the two components of post-vaccination tuberculin sensitivity. The results of the study indicate that post-vaccination tuberculin sensitivity can be used to assess BCG vaccination in areas with a high prevalence of non-specific sensitivity, provided the assessment is done in younger age groups and based on post-vaccination tuberculin sensitivity as measured at 2-3 months after vaccination and not later.