The present invention relates to the field of attenuated live viruses useful as vaccine or medicament for preventing or treating Porcine Reproductive and Respiratory Syndrome (PRRS) in swine, and is based on the surprising finding of a PRRS virus which is able to induce the interferon type I response of a cell infected by said virus. In one embodiment, the PRRS virus according to the invention is a PRRS virus mutant comprising, in comparison with the genome of a wild type strain, a mutation in the gene encoding the non structural protein 1 (nsp1) of said virus.
