The invention relates to methods for producing a propagation-competent strain of a mutant Reoviridae virus, and to a propagation-competent strain of a mutant Reoviridae virus that is obtainable by a method of the invention. The invention further relates to a propagation-competent strain of a mutant Reoviridae virus, comprising a deletion of a genetic region that is relevant for propagation of the virus, and to a vaccine, comprising a propagation-competent strain of a mutant Reoviridae virus.