Cross-protection conferred by intradermal PRRSV-1 modified live vaccine against a highly virulent Vietnamese PRRSV-2 strain in pigs.

Porcine reproductive and respiratory syndrome (PRRS) remain one of the most economically significant diseases affecting the global swine industry. Caused by the PRRS virus (PRRSV), the disease leads to reproductive failure in sows and respiratory disorders in pigs of all ages. In Vietnam, PRRSV was officially reported in 2007, and the emergence of highly pathogenic PRRSV (HP-PRRSV) strains has since intensified outbreaks with high morbidity and mortality. This study evaluated the efficacy of the intradermal PRRSV-1 commercial vaccine (UNISTRAIN® PRRS, HIPRA) in piglets experimentally challenged with a Vietnamese HP-PRRSV-2 strain. Twenty-eight PRRS naïve, 21-day-old piglets were randomly assigned to a VC group, including vaccinated and challenged pigs (n = 12); UVC group, including unvaccinated and challenged pigs (n = 12); and a UVUC group, including unvaccinated and unchallenged pigs (n = 4). At 5 weeks post-vaccination, all animals in VC group and UVC group were challenged intramuscularly with 3 mL of HP-PRRSV (lineage L8E, 2025; at a dose of 105 CCID₅₀/pig) and monitored for 28 days post-infection (dpi). Clinical signs, rectal temperature, viremia, antibody response, macro/micropathological lesions and growth performance were evaluated. Vaccinated pigs exhibited a lower incidence of fever and milder respiratory signs than unvaccinated pigs, corresponding to reduced clinical scores. Despite viremia in both groups post-infection, vaccinated pigs exhibited reduced viral loads, shorter viremia duration, and lower incidence than unvaccinated pigs at 7 dpi (p < 0.05). This constituted a significant reduction in viremia, evidenced by a 38.58% decrease in AUC. The antibody response developed earlier and persisted longer in vaccinated pigs, which displayed significantly higher S/P ratios at 7 dpi (p < 0.001). Lung lesions in vaccinated pigs were characterized by mild interstitial inflammation with preserved alveolar structure, whereas unvaccinated pigs exhibited severe, diffuse interstitial pneumonia with hemorrhage. Average daily gain was numerically higher in vaccinated animals, though not statistically significant (p > 0.05). Overall, intradermal PRRSV-1 vaccination partially protected against heterologous HP-PRRSV-2, lowering disease severity and promoting early antibody production.