Long duration of immunity against a type 1 heterologous PRRS virus challenge in pigs immunised with a novel PRRS MLV vaccine: a randomised controlled study.

Background: Porcine reproductive and respiratory syndrome virus (PRRSV) is widespread in commercial pig farms worldwide, and has a significant cost to the swine industry. Herd owners need a vaccine that will confer long-lasting immunity to prevent PRRSV infection and transmission. The studies described here evaluated duration of immunity conferred by a European-derived PRRS (isolate 94,881) modified live virus (MLV) vaccine, Ingelvac PRRSFLEX® EU, at 20, 24, and 26 weeks post-vaccination. Primary endpoints were the assessment of gross and histological lung lesions and viral RNA load in lung tissue 10 days following heterologous PRRSV challenge. Secondary endpoints included clinical observations, average daily weight gain (ADWG) and viral RNA load in serum 10 days post-challenge. Three blinded, vaccination-challenge efficacy studies were performed using separate cohorts of pigs ( n  = 56 per study). Pigs received either Ingelvac PRRSFLEX® EU (Group 1) or placebo (Groups 2 and 3). Groups 1 and 2 were subsequently challenged with heterologous European PRRSV isolate 205,817 at 20, 24 or 26 weeks post-vaccination.

Results: Mean gross lung lesion scores were significantly lower in Group 1 than in Group 2 at 24 and 26 weeks ( p  <  0.0001), but not at 20 weeks ( p  = 0.299). Significantly lower mean histological lung lesion scores were observed in Group 1 versus Group 2 at 20 ( p  = 0.0065), 24 ( p  <  0.0001) and 26 weeks ( p  <  0.0001). Mean viral RNA load in lung tissue was significantly lower in Group 1 than in Group 2 ( p  <  0.0001) at 20 ( p  <  0.0001), 24 ( p  <  0.0001) and 26 weeks ( p  <  0.0001). Cumulative viral RNA loads in serum during days 1-10 post-challenge were significantly lower in Group 1 than in Group 2 ( p  <  0.0001) in all studies. A significant increase in ADWG was observed in Group 1 compared with Group 2 at 20 weeks ( p  = 0.0027) and 24 weeks ( p  = 0.0004), but not at 26 weeks ( p  = 0.1041). There were no significant differences in clinical signs post-challenge in any study.

Conclusion: These results suggest that Ingelvac PRRSFLEX® EU confers long-term immunity to European heterologous PRRSV, which is maintained up to 26 weeks after vaccination, corresponding to the expected lifespan of commercial pigs.