PRRS modified live virus (MLV) vaccines have been licensed for clinical administration for over 20 years. The vaccines confer solid protection against homologous PRRSV strains. However, the extent and duration of protection against heterologous strains is highly variable and mostly sub-optimal. We recently generated a synthetic PRRSV strain that is able to confer unprecedented levels of protection. The goal of this renewal project is to attenuate the synthetic PRRSV strain and evaluate the safety and efficacy of the resulting attenuated PRRSV strain. The results of this project demonstrate that after attenuation, the synthetic PRRSV strain maintains the broadly protective phenotype of its parental strain.