Relation between pre-vaccination and post-vaccination tuberculin sensitivity. A contribution to the ecology of BCG vaccination.

BCG vaccination is commonly assessed in terms of post-vaccination sensitivity to tuberculin. If vaccination is followed by the development of a high degree of tuberculin sensitivity, it is assumed that the vaccination was successful. If, on the other hand, tuberculin sensitivity does not develop, it is assumed that the vaccination was unsuccessful. Both these assumptions equate post-vaccination tuberculin sensitivity with BCG-induced tuberculin sensitivity and disregard the possibility that environmental factors, such as the prevalence of low-grade naturally acquired tuberculin sensitivity, may affect the outcome of vaccination. Thus, while it seems reasonable to equate post-vaccination and BCG-induced tuberculin sensitivity in areas where low-grade sensitivity is uncommon, it might be unjustifiable to do so in areas where such sensitivity is prevalent.This paper analyses the relation between pre- and post-vaccination tuberculin sensitivity in a community with a high prevalence of naturally acquired low-grade tuberculin sensitivity. From this analysis it appears that post-vaccination tuberculin sensitivity may be only partly BCG-induced and cannot therefore be considered a reliable measure of the success of BCG vaccination in the presence of naturally acquired low-grade sensitivity. The author discusses the implications of this finding and concludes that a further evaluation of the protection afforded by BCG in areas where low-grade tuberculin sensitivity is prevalent is much needed.