Detection of PRRSV in the semen sample. Mean group log10 European or North American PRRSV RNA load (A) and mean group log10 TCID50/ml European or North American PRRSV (B) from the different treatment groups were determined. T01, boars immunized with modified live PRRSV vaccine and challenged with the European PRRSV; T02, boars immunized with modified live PRRSV vaccine and challenged with the North American PRRSV; T03, boars challenged with the European PRRSV only; T04, boars challenged with the North American PRRSV only. *, significant difference (P ≤ 0.001) between T02 and T04 at the same time point.