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PeerJ. 2016 Feb 11;4:e1513. doi: 10.7717/peerj.1513. eCollection 2016.

On the impact of masking and blocking hypotheses for measuring the efficacy of new tuberculosis vaccines.

Author information

1
Institute for Biocomputation and Physics of Complex Systems (BIFI), University of Zaragoza, Zaragoza, Spain; Department of Theoretical Physics, University of Zaragoza, Zaragoza, Spain.
2
Institute for Biocomputation and Physics of Complex Systems (BIFI), University of Zaragoza, Zaragoza, Spain; Sainte-Justine Hospital Research Centre, Montreal, Quebec, Canada; Department of Pediatrics, University of Montreal, Montreal, Quebec, Canada.
3
Department of Microbiology, Faculty of Medicine, University of Zaragoza, Zaragoza, Spain; CIBER Enfermedades Respiratorias, Instituto de Salud Carlos III, Madrid, Spain.
4
Department of Microbiology, Faculty of Medicine, University of Zaragoza, Zaragoza, Spain; CIBER Enfermedades Respiratorias, Instituto de Salud Carlos III, Madrid, Spain; Service of Microbiology, Miguel Servet Hospital, Zaragoza, Aragón, Spain.
5
Institute for Biocomputation and Physics of Complex Systems (BIFI), University of Zaragoza, Zaragoza, Spain; Department of Theoretical Physics, University of Zaragoza, Zaragoza, Spain; Complex Networks and Systems Lagrange Lab, Institute for Scientific Interchange, Turin, Italy.

Abstract

Over the past 60 years, the Mycobacterium bovis bacille Calmette-Guérin (BCG) has been used worldwide to prevent tuberculosis (TB). However, BCG has shown a very variable efficacy in different trials, offering a wide range of protection in adults against pulmonary TB. One of the most accepted hypotheses to explain these inconsistencies points to the existence of a pre-existing immune response to antigens that are common to environmental sources of mycobacterial antigens and Mycobacterium tuberculosis. Specifically, two different mechanisms have been hypothesized to explain this phenomenon: the masking and the blocking effects. According to masking hypothesis, previous sensitization confers some level of protection against TB that masks vaccine's effects. In turn, the blocking hypothesis postulates that previous immune response prevents vaccine taking of a new TB vaccine. In this work we introduce a series of models to discriminate between masking and blocking mechanisms and address their relative likelihood. We apply our methodology to the data reported by BCG-REVAC clinical trials, which were specifically designed for studying BCG efficacy variability. Our results yield estimates that are consistent with high levels of blocking (41% in Manaus -95% CI [14-68]- and 96% in Salvador -95% CI [52-100]-). Moreover, we also show that masking does not play any relevant role in modifying vaccine's efficacy either alone or in addition to blocking. The quantification of these effects around a plausible model constitutes a relevant step towards impact evaluation of novel anti-tuberculosis vaccines, which are susceptible of being affected by similar effects, especially if applied on individuals previously exposed to mycobacterial antigens.

KEYWORDS:

BCG; Blocking; Clinical trials; Environmental sensitization; Masking; TB vaccines; Tuberculosis

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