Recombinant modified vaccinia virus Ankara expressing antigen 85A (MVA85A). When used in heterologous prime-boost strategies, some viral vectors are very good at boosting previously primed T cell responses, particularly recombinant adenoviral constructs and recombinant pox viruses. Modified vaccinia virus Ankara (MVA) is an attenuated strain of vaccinia virus that has been passaged more than 500 times through chick embryo fibroblasts and as a consequence has lost both host range genes and cytokine receptor genes. MVA has an excellent safety record, as it was used to vaccinate more than 120,000 people at the end of the smallpox eradication campaign, with no serious adverse events. In humans, several hundred HIV negative volunteers have now been immunized with recombinant MVAs expressing a number of different antigens, with no serious adverse events. A recombinant MVA expressing an antigen from Plasmodium falciparum has been demonstrated to boost CD4+ and CD8+ T cells in humans. Recombinant MVAs are now in clinical trials for several infectious diseases including HIV, hepatitis B and malaria. Importantly, for a new TB vaccine, the safety of MVA as a viral vector has now been demonstrated in HIV positive patients.