A mutant virus for use as a vaccine for prophylaxis or therapy, wherein the genome of the virus is defective in respect of a gene essential for the production of infectious virus. In one aspect the mutant virus, e.g. a herpesvirus, e.g HSV-1 or HSV-2, is capable of protecting a susceptible species immunised therewith against infection by the corresponding wild-type virus. In another aspect, the mutant virus acts as a vector for an immunogenic protein derived from a pathogen, encoded by foreign DNA incorporated in the mutant virus. The mutant virus can be produced by a recombinant host cell which expresses a gene complementing the defect. The mutant virus can be infectious for the host to be protected, and the genetic defect can allow expression in the infected host of at least some of the viral genes, which can provoke a cell-mediated immune response. The defect can be in a glycoprotein gene such as gH.