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Rose and colleagues have demonstrated that a modified-live vaccine is able to decrease considerably Porcine reproductive and respiratory syndrome virus (PRRSv) spread in experimental conditions. Twelve piglets, vaccinated against PRRSv at 3 weeks of age (Porcilis PRRS ID®, MSD), were inoculated at 31 days post-vaccination with a heterologous genogroup 1.1 strain, and put in contact with 12 vaccinated piglets during 49 days. Piglets were monitored for clinical symptoms and individual blood samples were taken twice a week. In inoculated piglets, the genome viral load specific to the inoculated strain was reduced and viraemia shortened in vaccinated piglets. In contact pigs, the challenge strain was barely detected in the serum whereas it was detected in all contact non-vaccinated piglets. They estimated that transmission rate was 10-times lower in vaccinated than in non-vaccinated piglets and that the duration of infectiousness was reduced, leading to a reproduction ratioEH less than 1 versus 5.4 in non vaccinated piglets.