In Denmark, a porcine reproductive and respiratory syndrome virus (PRRSV) control programme, comprising vaccination of seropositive herds with a live American type PRRSV vaccine, was started in 1996. In several of these herds, spread of vaccine virus from vaccinated 3-18 week old pigs to non-vaccinated sows was demonstrated by the isolation of vaccine virus from fetuses and stillborn piglets. Surprisingly, sows infected with the American type vaccine strain consistently exhibited significantly stronger serological responses towards European type PRRSV than American type PRRSV. In order to elucidate whether the unexpectedly strong serological reaction towards European-type PRRSV in American type PRRSV infected sows was due to a booster reaction, or reactivation of an unrecognized, latent infection in the sows with European type PRRSV, a challenge study with the vaccine was carried out. In this study, the stronger serological response towards European type PRRSV than towards American type PRRSV was reproduced, and reactivation of the previous natural infection with European PRRSV could neither be demonstrated by virus isolation nor by RT-PCR. So, the increase in antibody titers towards European PRRSV in previously European PRRSV infected pigs after challenge with the vaccine strain seems to be the result of a boosting effect on the immune system, induced by the heterologous vaccine PRRSV strain.