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Introduction
More than a century after Mycobacterium tuberculosis was first identified and decades after antibiotic therapies were initiated, tuberculosis (TB) still remains a leading cause of morbidity and mortality in humans and a major public health problem worldwide [1] . With an estimated one-third of the world's population being infected by M. tuberculosis and 1.5 million deaths resulting from TB infections each year, new control measures are needed to combat this devastating disease. The only licensed vaccine against M. tuberculosis, live Mycobacterium bovis BCG, is one of the most widely used global vaccines, with more than 3 billion people having been immunized during its more than 8 decades of use [2] . BCG vaccine is relatively safe and is easy and inexpensive to produce. However, estimates of the level and persistence of protection against TB induced by BCG immunization have been highly variable. While randomized controlled trials and retrospective case control studies have shown that BCG immunization is effective in reducing cases of severe disseminated disease in children, the efficacy of BCG in preventing tuberculosis in adults has ranged from 0 to 80% [3,4] . Furthermore, the longevity of BCG-induced protection is uncertain and may be population dependent [5] . Although it has been estimated that BCG's efficacy wanes about 10 years after vaccination, a 60 year follow-up of a 1930s clinical trial showed that BCG was still 52% effective 6 decades after immunization of native Americans [6] . Interestingly, several reports have also indicated that BCG vaccination may impart wide ranging health-related effects that are unrelated to its anti-tuberculosis activity including an overall beneficial impact on childhood survival [7,8] .
Since BCG is widely used in countries with high TB burdens and BCG vaccine does reduce the incidence of severe childhood extrapulmonary TB disease, it is unlikely that BCG vaccine will be replaced in the near future. However, many investigators are developing new strategies to improve the protective activity of BCG [9,10] . One approach has been to amplify BCG-induced immune responses by boosting with subunit or viral vaccines after an initial BCG priming vaccination. A potentially simpler and less expensive alternative approach to increase BCG's immunogenicity is to formulate BCG in lipid-containing mixtures. Lipid encapsulation has been shown in mice, deer, possums,...