M protein-specific serum antibody increased steadily from the second to the fourth DNA vaccination with the pVAX1-Ub-M vector which drives M protein expression in vivo (Additional file 5, Fig. S5 ), indicating that DNA vaccination elicited M protein-specific immune responses as expected. A subsequent booster vaccination with the recombinant vaccinia virus, rWR-PRRSV-M, elicited a further increase in PRRSV-specific antibody (P < 0.01, Additional file 5, Fig. S5 ). In contrast, control mice vaccinated with pVAX1-only or PBS-only did not show significant increases in M protein-specific antibody titers after the booster vaccination with rWR-PRRSV-M (P > 0.05, Additional file 5, Fig. S5 ). Overall, mice vaccinated with pVAX1-Ub-M generated significantly higher M protein-specific antibody titers compared to mice vaccinated with pVAX1 or PBS (P < 0.01, Additional file 5, Fig. S5 ). These results indicate that rWR-PRRSV-M amplifies the protective effects of DNA vaccination and reveals the advantage of this priming-boosting strategy. DNA vaccination with pVAX1-Ub-M likely drives the differentiation of memory B cells which are subsequently activated by rWR-PRRSV-M following the booster immunization, resulting in increased M-specific antibody titers. Mice vaccinated with PBS and pVAX1 would not be expected to generate M protein-reactive memory B cells, accounting for a less pronounced increase in M protein-specific antibody titers following rWR-PRRSV-M innoculation.