Genomic characteristics and pathogenicity of three newly isolated NADC34-like PRRSV strains in China and evaluation of one strain for inactivated vaccine candidate

Porcine reproductive and respiratory syndrome (PRRS) has caused substantial economic losses to the pig industry. Recently, the emergence of NADC34-like PRRS virus (PRRSV) has made the worldwide circulation of PRRSV more complex. In 2017, the first detection of NADC34-like PRRSV in China was reported, but the pathogenicity and biological characteristics of this novel PRRSV variant remain to be further elucidated. In this study, we conducted analyses of the whole genome sequence and pathogenicity of three NADC34-like PRRSV strains isolated in China. The three strains, designated HeB-05, SD-02, and JL-1152, were isolated using porcine alveolar macrophages (PAMs). However, only HeB-05 demonstrated robust replication in Marc-145 cells, achieving a high viral titer of up to 10^7.57 TCID₅₀/mL. The genomes of SD-02 and JL-1152 have undergone recombination, while no recombination events were detected in the genome of HeB-05. Artificial infection tests revealed that SD-02 and JL-1152 exhibited relatively high virulence to piglets, causing significant fever and mortality, whereas HeB-05 did not cause death in piglets. Following assessments of the immunogenic efficacy of an inactivated vaccine developed using HeB-05 in pig models showed the vaccine provided partial protection against challenge with the parental strain. This study introduces a novel candidate vaccine strain for NADC34-like PRRSV, which holds significant implications for the prevention and control of PRRS.