This series of Phase I studies have also replicated in The Gambia, and the results from The Gambia are equally promising. In both the UK and The Gambian studies, MVA85A induces 5-10 fold higher immune responses than any other recombinant MVA in clinical trials. The most likely explanation for this is that the volunteers have some weak pre-existing anti-mycobacterial immunity induced by exposure to environmental mycobacteria, and this specific anti-mycobacterial immunity is being boosted by MVA85A. We have some preliminary data to support this hypothesis, and other groups have also found pre-existing immunity to environmental mycobacteria in healthy UK adults. Vaccines that are good at boosting previously primed T cell responses, such as MVA85A, are potentially good candidates for a post-exposure vaccine designed to boost the immune response and prevent the development of disease in those latently infected. If the high levels of specific T cells seen after a single immunisation with MVA85A are attributable to the boosting of pre-existing environmentally induced responses, then this makes MVA85A an extremely promising candidate for a boosting vaccine, given as a post-exposure vaccine in latent infection.