Humoral immune responses and viral shedding following vaccination with modified live porcine reproductive and respiratory syndrome virus vaccines.

The antibody response and pattern of shedding of vaccine virus following vaccination with modified live genotype I or II porcine reproductive and respiratory syndrome virus (PRRSV) vaccines (MLVs) were investigated. Ninety PRRSV-free pigs were divided randomly seven, groups including the NEG, EU1, EU2, US1, US2, US3 and US4 groups. The NEG group was unvaccinated. The EU1, EU2, US1, US2, US3 and US4 groups were vaccinated with the following MLVs: AMERVAC® PRRS, Porcillis® PRRS, Fostera™ PRRS, Ingelvac® PRRS MLV, Ingelvac® PRRS ATP, and PrimePac™ PRRS+ , respectively. Sera were quantitatively assayed for viral RNA using qPCR. Antibody responses were measured using Idexx ELISA and serum neutralization (SN). Shedding of vaccine virus was investigated using sentinel pigs and by detection of viral RNA in tonsil scrapings. Antibody responses were detected by ELISA at 7-14 days post-vaccination (DPV) and persisted at high titers until 84 DPV in all MLV groups. The SN titers were delayed and isolate-specific. SN titers were higher for the homologous virus than for heterologous viruses. Age-matched sentinel pigs introduced into the EU2, US2 and US3 groups at 60 DPV seroconverted. In contrast, sentinel pigs introduced at 84 DPV remained negative in all of the MLV groups. Vaccine viral RNA was detected in tonsil scrapings from the EU2, US2 and US3 groups at 84-90 DPV. No viral RNA was detected beyond 70 DPV in the EU1, US1 and US4 groups. In conclusion, all MLV genotypes induced rapid antibody responses, which were measured using ELISA. The development of SN antibodies was delayed and isolate-specific. However, the shedding pattern was variable and depended on the by virus isolate used to manufacture the vaccine.