BCG vaccination protects against infection with Mycobacterium tuberculosis ascertained by tuberculin skin testing

Objectives: There is conflicting evidence as to whether Bacillus Calmette–Guérin (BCG) vaccination offers protection against Mycobacterium tuberculosis infection ascertained by a positive tuberculin skin test (TST). We investigated the association between BCG vaccination status and TST results in a set of surveys at increasing TST cut-off values to take cross-reactions to BCG vaccination into account. Methods: Secondary analysis of data from three consecutive tuberculin surveys done among schoolchildren in Tanzania between 1990 and 2002. BCG vaccination status was ascertained by the presence of a typical scar. Results: We analyzed data of 277,588 children of whom 77.7% were BCG vaccinated and 8.5% had TST indurations ≥ 15 mm, 5.1% ≥ 17 mm and 2.8% ≥ 19 mm. In the combined analysis, odds ratios for a positive TST were > 1 for children with BCG up to TST cut-off values of 16 mm. For cut-off values > 17 mm crude and adjusted odds ratios were significantly < 1, and decreased with further increasing cut-off values. Conclusions: Using a methodology that makes use of the differences in TST reaction sizes between specific and non-specific responses, we showed that BCG vaccination was associated with reduced prevalence of M. tuberculosis infection as measured by the tuberculin skin test, suggesting a protective effect.