Influence of maternally-derived antibodies on live attenuated influenza vaccine efficacy in pigs.

Vaccination during pregnancy is practiced in swine farms as one measure to control swine influenza virus (SIV) infection in piglets at an early age. Vaccine-induced maternal antibodies transfer to piglets through colostrum and stabilize the herd: however, maternally derived antibodies (MDA) interfere with immune response following influenza vaccination in piglets at the later stage of life. In addition, MDA is related to enhanced respiratory disease in SIV infection. Previously, we have developed a bivalent live attenuated influenza vaccine (LAIV) which harbors both H1 and H3 HAs. We demonstrated vaccination of this LAIV provided protection to homologous and heterologous SIV infection in pigs. In this study we aimed to investigate the influence of MDA on LAIV efficacy. To this end, SIV sero-negative sows were vaccinated with a commercial vaccine. After parturition, nursery piglets were vaccinated with LAIV intranasally or intramuscularly, and were then challenged with SIV. We report that MDA hampered serum antibody response induced by intramuscular vaccination but not by intranasal vaccination of the LAIV. Viral challenge in the presence of MDA caused exacerbated respiratory disease in unvaccinated piglets. In contrast, all LAIV vaccinated piglets were protected from homologous viral infection regardless of the route of vaccination and the presence of MDA. Our results demonstrated that LAIV conferred protection in the presence of MDA without inciting exacerbated respiratory disease.