The need for new vaccine against tuberculosis Tuberculosis (TB) kills about three million people annually. It is estimated that one third of the world's population are latently infected with Mycobacterium tuberculosis (M.tb) (Dye, 1999). These latently infected individuals are at risk of reactivation of infection, should they become immunosuppressed. Worldwide, coinfection with HIV is the commonest cause of immunosuppression and increases the chances of reactivation from a 10% lifetime risk to a 10% annual risk (Corbett, 1996). The currently available vaccine, M. bovis BCG, is largely ineffective at protecting against adult pulmonary disease in endemic areas and it is widely agreed that a new more effective tuberculosis vaccine is a major global public health priority (Colditz, 1994). However, it may be unethical and impractical to test and deploy a vaccine strategy that does not include BCG, as BCG does confer worthwhile protection against TB meningitis and leprosy. An immunisation strategy that includes BCG is also attractive because the populations in which this vaccine candidate will need to be tested will already have been immunised with BCG. Given the high prevalence of infection with M.tb, a vaccine that could be administered to latently infected individuals and eradicate latent infection would have an enormous impact on the mortality and morbidity from TB. M.tb is an intracellular organism. CD4+ Th1-type cellular responses are essential for protection and there is increasing evidence from animal and human studies that CD8+ T cells also play a protective role (Flynn, 2001). However, it has generally been difficult to induce strong cellular immune responses in humans using subunit vaccines. DNA vaccines, recombinant viral vectors and protein/adjuvant combinations all induce both CD4+ and CD8+ T cells, however none of these antigen delivery systems induce high levels of antigen specific T cells, when used alone.