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Over the past 60 years, the Mycobacterium bovis bacille Calmette-Gu\'erin(BCG) has been used worldwide to prevent tuberculosis (TB). However, BCG hasshown a very variable efficacy in different trials, showing a wide range ofprotection in adults against pulmonary TB. Previous studies indicate that thisfailure is related to pre-existing immune response to antigens that are commonto environmental sources of mycobacterial antigens and Mycobacteriumtuberculosis. Specifically, two different mechanisms have been hypothesized:the masking, (previous sensitization confers some level of protection againstTB), and the blocking (previous immune response prevent vaccine taking of a newTB vaccine), effects. In this work we introduce a series of models todiscriminate between masking and blocking mechanisms and address their relativelikelihood. The application of our models to interpret the results coming fromthe BCG-REVAC clinical trials, specifically designed for the study of sourcesof efficacy variability yields estimates that are consistent with high levelsof blocking (41% in Manaus -95% C.I. [14%-68%]- and 96% in Salvador -95% C.I.[52%-100%]-), and no support for masking to play any relevant role in modifyingvaccine efficacy either alone or aside blocking. The quantification of theseeffects around a plausible model constitutes a relevant step towards impactevaluation of novel anti-tuberculosis vaccines, which are susceptible of beingaffected by similar effects if applied on individuals previously exposed tomycobacterial antigens.

On the impact of Masking and Blocking Hypotheses for measuring efficacy of new tuberculosis vaccines