Correlates of protective immunity to Mycobacterium tuberculosis in humans are desirable for identifying protective antigens, demonstrating the immunogenicity of a vaccine candidate and its potential efficacy, and permitting optimization of the dose, vehicle, adjuvant, and schedule of immunization. Potential correlates can be proposed on the basis of animal models and ex vivo/in vitro studies in humans. Most critical is their validation; ultimate validation will require correlation with protection in a phase III efficacy trial of an effective vaccine. Other approaches, however, can allow selection of the most promising correlates for inclusion in phase I and II and, ultimately, phase III vaccine trials. Current data from experimental models and studies of patients with pulmonary tuberculosis and their household contacts indicate that Mycobacterium tuberculosis-stimulated whole-blood production of interferon-γ, although imperfect, is the best available correlate. Nonetheless, further refinement of this assay and additional studies of more complex assays that model M. tuberculosis killing and cytotoxic T lymphocyte activity are warranted. During planning of a vaccine trial, the best available correlates of immunity can be selected for inclusion.
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