The invention provides isolated polynucleotide molecules that comprise a DNA sequence encoding an infectious RNA sequence encoding a genetically-modified North American PRRS virus, methods to make it and related polypeptides, polynucleotides, and various components. Vaccines comprising the genetically modified virus and polynucleotides and a diagnostic kit to distinguish between naturally infected and vaccinated animals are also provided.

Provided are a chimeric virus expressing a porcine reproductive and respiratory syndrome virus (PRRSV)-derived peptide and a use thereof as a vaccine. The chimeric virus has an excellent immune stimulating effect and is useful as a vaccine that can effectively protect against PRRSV by suppressing viral amplification in target cells.

The present invention is related to improved modified live PRRS vaccines containing new PRRSV European strains of PRRSV and methods of use and manufacture of such vaccines.