Live or modified live PRRS vaccines for administration to swine are provided which are of low virulence and confer effective immunity against PRRS. The preferred vaccines include virus isolates having average plaque diameters of less than about 2 mm and low pathogenicity. A preferred vaccine includes a small plaque diameter strain, ATCC Accession No. VR2509. The vaccines of the invention may be administered to breeding females or gilts and to weaned piglets, and is effective to immunize the swine against both the respiratory and reproductive forms of the disease.