[Risk of tubercular infection among schoolchildren with high BCG coverage]

Mycobacterium tuberculosis infections may be difficult to detect by the tuberculin test in populations where nonspecific tuberculin sensitivity (induced by BCG and "atypical" mycobacteria) is frequent. In those populations, the rate of M. tuberculosis infection can be determined by comparing the levels of sensitivity before and after BCG vaccination, and classifying as infected those who do not show increased sensitivity in the post-vaccination test. To test this method on a BCG-vaccinated population, a survey was carried out in a sample of 1,721 schoolchildren 6 to 10 years of age, from a suburban area of the city of Rio de Janeiro, where BCG vaccination had been carried out on a large scale. The results suggest that the method is valid for BCG-vaccinated populations, although its accuracy may be affected by the intrinsic variability of tuberculin tests and BCG vaccination, by the action of infections with non-tuberculous mycobacteria, and by low infection rates. The tuberculous infection rate found in the sample group (4.13%) was compared with the results of a tuberculin survey carried out in the same area in 1970, and the incidence of infection was found to be decreasing by 7.68% annually. The annual risk of tuberculous infection for 1986 was 0.36 or 0.51%, depending on the mathematical model used for the calculation. These figures are smaller than the official estimates derived from the incidence rates of notified cases or from surveys made before intradermal vaccination was adopted. This may be due to methodological error, inaccuracies of notification data, association between age and risk of infection and/or peculiarities of the surveyed area.