The present invention provides an attenuated virus, which is derived from Modified Vaccinia Ankara virus and characterized by the loss of its capability to reproductively replicate in human cell lines. 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. In addition, a method is provided for the administration of a therapeutically effective amount of the virus, or its recombinants, in a vaccinia virus prime/vaccinia virus boost innoculation regimen. The present invention relates to a method of virus amplification in primary cells which are cultivated in a serum free medium. Viruses produced by this method are advantageously free of any infectious agents comprised in animal sera.