Vaccination against porcine reproductive and respiratory syndrome virus (PRRSV) reduces the magnitude and duration of viremia following challenge with a virulent heterologous field strain

Forty PRRS-negative, three week-old weaned pigs were randomized into two groups in separate rooms and inoculated with a modified live PRRS vaccine (Fostera® PRRS) or control (PBS). Four weeks after vaccination pigs were rehoused in a single room and challenged intranasally and intramuscularly with virulent PRRSV strain NADC20. Timed serum samples were collected and titrated for PRRS virus and anti-PRRS virus antibodies. The study concluded when ≥80% of the pigs in the control group were determined to be virus negative (27 days post-challenge). Mean duration of viremia was significantly lower (p = 0.0327) for vaccinated pigs compared to non-vaccinated pigs. A significant reduction (p ≤ 0.0053) in mean post-challenge viremia titer was seen in vaccinates compared to non-vaccinates from days 8 through 22 post-challenge. At the individual pig level, no pigs in the vaccinated group had detectible PRRSV in serum at the end of the study (27 days post-challenge), while 15% of non-vaccinated pigs remained positive for virus.