MVA is a highly attenuated strain of vaccinia virus that is unable to replicate efficiently in human cell lines and most mammalian cells. Viral replication is blocked at a late stage of virion assembly, so, importantly, viral and recombinant protein synthesis is unimpaired. This means that MVA is an efficient single round expression vector, incapable of causing infection in mammals. Replication-deficient recombinant MVA has been seen as an exceptionally safe viral vector. This safety in man is consistent with the avirulence of MVA in animal models, where recombinant MVAs have also been shown to be protectively immunogenic as vaccines against viral diseases and cancer. Importantly for a vaccine which may eventually be used in a large proportion of the population, recombinant MVAs expressing HIV antigens have been shown to be safe and immunogenic in HIV-infected subjects.