Exogenous cDNA capable of expressing interferon″ activity, exogenous interferon″ protein, inducers of endogenous interferon″ protein activity, inducers of endogenous interferon $ protein activity, inducers of endogenous interferon′ activity, or inducers of other immune-enhancing activity can be combined with a vaccine to enhance an immune response. Specifically disclosed are adjuvant and vaccine combinations where the adjuvant comprises a cDNA capable of expressing interferon″ activity, a complex comprising polyriboinosinic-polyribocytidilic acid, or a complex comprising polyriboinosinic-polyribocytidilic acid, poly-L-lysine, and carboxymethylcellulose and where the vaccine is a live vaccine virus derived from a virus causing porcine reproductive and respiratory syndrome disease.