Effects of infection with atypical mycobacteria on BCG vaccination and tuberculosis.

Attempts to assess the antituberculous effects of BCG vaccination in human populations have produced strikingly different results. The sharpest contrast appears in the large controlled studies carried out by the British Medical Research Council (1) and by the U.S. Public Health Service (2, 3). In the British study, the tuberculosis morbidity rate among unvaccinated control subjects was between four and five times higher than in comparable vaccinated groups. In the Public Health Service studies, the rates were only slightly higher in control subjects than in vaccinated groups. Acceptance of the British results would lead to the view that vaccination against tuberculosis is a valuable tool in tuberculosis control. Acceptance of the Public Health Service results naturally leads to an opposing conclusion. Various attempts have been made to reconcile the results of these and other vaccination studies. One explanation is that the differences are related to infections with atypical mycobacteria. Knowledge of these infections is still limited, but enough has been done in both human and laboratory studies (4-12) to demonstrate that infection with these organisms induces, with differing frequency and intensity, nonspecific (cross or heterologous) tuberculin reactions. Also, laboratory studies (13-19) show, and human studies (1-3, 20) suggest that infections with atypical mycobacteria create some capacity-varying markedly for the different mycobacteria-to modify the course of virulent tuberculous disease. Infection with these organisms constitutes a kind of natural vaccination that resembles vaccination with BCG. The present paper is a report of results from a laboratory experiment in which an attempt was made to reproduce in guinea pigs what most likely occurs when BCG vaccination is