Influenza A virus hemagglutinin protein subunit vaccine elicits vaccine-associated enhanced respiratory disease in pigs.

Vaccine-associated enhanced respiratory disease (VAERD) can occur when pigs are challenged with heterologous virus in the presence of non-neutralizing but cross-reactive antibodies elicited by whole inactivated virus (WIV) vaccine. The aim of this study was to compare the effects of heterologous δ1-H1N2 influenza A virus (IAV) challenge of pigs after vaccination with 2009 pandemic H1N1 virus (H1N1pdm09) recombinant hemagglutinin (HA) subunit vaccine (HA-SV) or temperature-sensitive live attenuated influenza virus (LAIV) vaccine, and to assess the role of immunity to HA in the development of VAERD. Both HA-SV and LAIV vaccines induced high neutralizing antibodies to virus with homologous HA (H1N1pdm09), but not heterologous challenge virus (δ1-H1N2). LAIV partially protected pigs, resulting in reduced virus shedding and faster viral clearance, as no virus was detected in the lungs by 5 days post infection (dpi). HA-SV vaccinated pigs developed more severe lung and tracheal lesions consistent with VAERD following challenge. These results demonstrate that the immune response against the HA protein alone is sufficient to cause VAERD following heterologous challenge.