Protective Efficacy of a Candidate Live-Attenuated Vaccine Derived from the SD-R Strain against NADC34-like Porcine Reproductive and Respiratory Syndrome Virus

NADC34-like porcine reproductive and respiratory syndrome virus (PRRSV) strains were first detected in China in 2017 and became major circulating strains in 2021. Our previous study showed that the live-attenuated vaccine candidate SD-R strain could provide broad cross-protection against different NADC30-like PRRSVs (sublineage 1.8). However, the protective effect of SD-R against NADC34-like PRRSV is unclear. Here, a novel NADC34-like PRRSV, LNTZJ1341-2012, was isolated from a pig farm experiencing disease in 2020. Sequence analysis revealed that LNTZJ1341-2012 belonged to PRRSV-2 sublineage 1.5, exhibited the same Nsp2 amino-acid deletion characteristics as IA/2014/NADC34, and had not recombined with other strains. Additionally, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative NADC34-like strain (LNTZJ1341-2012). The control piglets in the challenge experiment displayed clinical signs typical of PRRSV infection, including transient fever, high viremia, mild clinical symptoms, and histopathological changes in the lungs and submaxillary lymph nodes. In contrast, SD-R vaccination significantly reduced serum and lung tissue viral loads, and vaccinated piglets did not show any clinical symptoms or histopathological changes. Our results demonstrated that LNTZJ1341-2012 is a mildly virulent NADC34-like PRRSV and that the live-attenuated vaccine SD-R can prevent the onset of clinical signs upon challenge with the NADC34-like PRRSV LNTZJ1341-2012 strain, indicating that SD-R is a promising vaccine candidate for the swine industry.