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Eliciting long-term protection against virus-induced diseases requires the development of attenuated viruses and efficient vectors.In some cases,this is possible because the live modified virus does not have dominant components that interfere with the induction of a protective immune response,as we will describe for the severe and acute syndrome (SARS) virus (SARS-CoV).In contrast,in other cases,such as for the porcine respiratory and reproductive syndrome virus (PRRSV),this is a less clear possibility as a consequence of the presence of many viral proteins that inhibit the onset of a protective immune response.