Introduction Modied Vaccinia virus Ankara (MVA) is a replication-decient attenuated poxvirus that was derived from Vaccinia virus by Mayr et al. through more than 500 blind passages in chick embryo broblast (CEF) cell culture . With the notable exception of the Syrian hamster cell line BHK-21 , MVA is unable to replicate productively in mammalian cells, though genome replication, late gene expression and immature virion formation usually occur , . During its attenuation, MVA acquired large genomic deletions totalling about 30 kb, resulting in the complete loss of 26 open reading frames (ORFs), together with truncation or fragmentation of a further 21 ORFs and numerous smaller scale mutations . MVA does not therefore express many of the known poxviral immune evasion and virulence factors –, though how this determines its abortive phenotype in mammalian cells is not well understood . Despite these features, MVA is at
least as immunogenic as conventional replicating Vaccinia virus – and has the advantage of a considerably improved safety prole .