Exogenous cDNA capable of expressing interferon .alpha. activity, exogenous interferon .alpha. protein, inducers of endogenous interferon .alpha. protein activity, inducers of endogenous interferon .beta. protein activity, inducers of endogenous interfereon .GAMMA. 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 .alpha. 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.