PORCINE circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) are the two most important pathogens currently responsible for major economic losses to the swine industry worldwide. PCV2 vaccines are extensively used and are able to reduce the clinical signs of PCV2-associated disease. The efficacy of PCV2 vaccination likely depends on both humoral and cellular immunity that can either be maternally acquired (sow vaccination) or actively induced (piglets vaccination) (Fort and others 2008, 2009, Opriessnig and others 2010, Martelli and others 2011). Taking into account that PRRSV suppresses innate immunity and influences the development and the activation of the specific immune response (Butler and others 2014), this study is aimed at evaluating the effects of PRRSV viraemia on the immune response to PCV2 vaccination in four-week-old piglets. Indeed, PCV2 vaccination in two- to four-week-old piglets is extensively applied where PCV2 infection and associated disease are present. Two hundred piglets from a 1000 farrow-to-wean PRRSV infected and unstable sows herd were divided into two treatment groups (100 animals each); one group was vaccinated at four weeks of age with a PCV2 commercial vaccine (Porcilis PCV – MSD) and the second group was not vaccinated. The animals were assigned to the different groups as they came to hand …