A modified-live PRRSV vaccine administered at weaning confers partial protection against a highly virulent PRRSV-1 strain throughout the grower-finisher period.

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically important diseases affecting the swine industry. In 2020, a highly virulent PRRSV-1 strain, commonly known as Rosalía, emerged in Spain, causing significant production losses and raising concerns regarding the level of protection provided by commercially available vaccines. This study evaluated the duration of immunity conferred by the modified-live virus vaccine Porcilis® PRRS following vaccination at weaning and subsequent heterologous challenge with a highly virulent PRRSV-1 Rosalía-like strain (Lleida 029_22) during the grower-finisher period. Vaccinated animals exhibited milder clinical signs, a shorter duration of pyrexia, reduced lung lesions and maintained average daily weight gain (ADWG) following challenge, whereas non-vaccinated animals showed an approximate 50% reduction in ADWG. Vaccination also significantly reduced viremia, viral shedding via the nasal, oral and rectal routes, and viral loads in tissues at necropsy. However, PRRSV infection and viral shedding were not completely prevented, remaining detectable in vaccinated pigs until the end of the study. Vaccinated pigs also developed earlier and more sustained PRRSV-specific antibody responses than non-vaccinated animals. Overall, these findings demonstrate that Porcilis® PRRS confers partial long-term protection against a highly virulent heterologous PRRSV-1 Rosalía-like strain throughout the grower-finisher period, supporting vaccination at weaning as an effective strategy to mitigate the impact of PRRSV infection later in production.