Live Attenuated Viruses are useful as a vaccine or medicine to prevent or treat porcine Reproductive and Respiratory Syndrome (PRRS) in pigs, and is based on the finding of PRRS virus is able to induce Type I interferon Response of a cell infected by the virus.In One implementation, the PRRSV PRRSV Mutant virus is a virus which comprises, in comparison with the Genome of a wild-type Strain, a mutation in the gene encoding the non Structural Protein 1 (NSP1) of the virus.Claim 38: Use PRRSV of any of claims 1 to 37, wherein is for the preparation of a vaccine or medicine for prophylaxis or treatment of porcine Reproductive and Respiratory Syndrome preferably in pigs.Claim 39: a method for the prophylaxis or treatment of porcine Reproductive and Respiratory Syndrome, characterized in that it comprises Administering the PRRSV of any of claims 1 to 37, to an animal, preferably a Pig, more preferably a Pig In particular, preferably to a Pig, a Pig.Claim 44: Medicine or vaccine, characterized in that it comprises the PRRSV of any of claims 1 to 37.Claim 57: a method to induce an immune response against PRRSV or to Treat or prevent PRRSV or to prevent or reduce viremia of PRRSV and / or clinical symptoms, particularly the High body temperature,Associated with the infection of PRRSV or for the Prevention or reduction of elevated body temperature associated with the Administration of a vaccine of PRRSV attenuated to an animal, characterized in that it comprises the step of administering immunogenic composition according to any of L As demands 51 to 54 to an animal in need.Virus vivos atenuados que son útiles como vacuna o medicamento para prevenir o tratar el síndrome reproductivo y respiratorio porcino (PRRS) en cerdos, y se basa en el hallazgo de un virus PRRS que es capaz de inducir la respuesta del interferón tipo I de una célula infectada por dicho virus. En una realización, el virus PRRS es un virus PRRS mutante que comprende, en comparación con el genoma de una cepa
