The present invention provides an attenuated virus, which is derived from Modified Vaccinia Ankara virus and characterized by induction of protective immunity against a lethal vaccinia virus infection in a mouse, whereby the effective amount of the MVA-BN virus, or a derivative thereof, required to confer said immunity is less than the effective amount of MVA-575 required to render said mouse immune to a lethal vaccinia virus infection, and/or the MVA-BN virus, or a derivative thereof, induces at least substantially the same level of immunity in vaccinia virus prime/vaccina virus boost regimes when compared to DNA prime/vaccinia virus boost regimes. It further describes recombinant viruses derived from this virus and the use of the virus, or its recombinants, as a medicament or vaccine. A method is provided for inducing an immune response in individuals who may be immune-compromised, receiving antiviral therapy, or have a pre-existing immunity to the vaccine virus.