Efficacy Evaluation of a VR-2332-Based Modified Live Vaccine Against NADC30-like PRRSV in China

<b>Background:</b> Porcine reproductive and respiratory syndrome is caused by PRRSV. Modified live vaccines (MLVs) are widely used to control PRRSV infection, but their efficacy against the emerging NADC30-like variant remains unclear. This study aimed to evaluate the efficacy of a VR-2332-based MLV against the NADC30-like PRRSV strain HNjz15. <b>Methods:</b> Forty piglets were randomized into a vaccination group (MLV group), negative control group (NC group), and sentinel group. MLV group piglets were immunized with a commercial MLV at 3 weeks of age and challenged with HNjz15 (10^6.6 TCID₅₀/mL) at 21 days post-immunization. Clinical symptoms, viral load, antibody responses, cytokine levels, and lung lesions were monitored for 14 days post-challenge. <b>Results:</b> Although fever and respiratory symptoms were more pronounced in the NC group pigs than those of the MLV group (average percent occurrence: 65.2% vs. 52.9%), there was no statistical difference (<i>p</i> > 0.05) in the occurrence of respiratory symptoms between the two groups from 5 dpc. Reduced weight gains (by 40-53%) were also observed in the MLV and NC groups compared with the sentinels. The MLV and NC groups exhibited severe lung lesions, while there was no marked difference in viral RNA loads in serum and tissue samples between the MLV and NC groups (<i>p</i> > 0.05). The MLV vaccine induced a significant high level of N protein-specific antibodies compared to the NC group. There was also no significant difference in IFN-γ or TNF-α response to the HNjz15 challenge in both groups (<i>p</i> > 0.05). <b>Conclusions:</b> The VR-2332-based MLV does not provide adequate protection against challenge with the PRRSV-2 NADC30-like strain HNjz15.