The immune response and maternal antibody interference to a heterologous H1N1 swine influenza virus infection following vaccination.

This study investigated the efficacy of a bivalent swine influenza virus (SIV) vaccine in piglets challenged with a heterologous H1N1 SIV isolate. The ability of maternally derived antibodies (MDA) to provide protection against a heterologous challenge and the impact MDA have on vaccine efficacy were also evaluated. Forty-eight MDA(+) pigs and 48 MDA(-) pigs were assigned to 8 different groups. Vaccinated pigs received two doses of a bivalent SIV vaccine at 3 and 5 weeks of age. The infected pigs were challenged at 7 weeks of age with an H1N1 SIV strain heterologous to the H1N1 vaccine strain. Clinical signs, rectal temperature, macroscopic and microscopic lesions, virus excretion, serum and local antibody responses, and influenza-specific T-cell responses were measured. The bivalent SIV vaccine induced a high serum hemagglutination-inhibition (HI) antibody titer against the vaccine virus, but antibodies cross-reacted at a lower level to the challenge virus. This study determined that low serum HI antibodies to a challenge virus induced by vaccination with a heterologous virus provided protection demonstrated by clinical protection and reduced pneumonia and viral excretion. The vaccine was able to prime the local SIV-specific antibody response in the lower respiratory tract as well as inducing a systemic SIV-specific memory T-cell response. MDA alone were capable of suppressing fever subsequent to infection, but other parameters showed reduced protection against infection compared to vaccination. The presence of MDA at vaccination negatively impacted vaccine efficacy as fever and clinical signs were prolonged, and unexpectedly, SIV-induced pneumonia was increased compared to pigs vaccinated in the absence of MDA. MDA also suppressed the serum antibody response and the induction of SIV-specific memory T-cells following vaccination. The results of this study question the effectiveness of the current practice of generating increased MDA levels through sow vaccination in protecting piglets against disease.