Vaccines that reduce viral shedding do not prevent transmission of H1N1 pandemic 2009 swine influenza a virus infection to unvaccinated pigs

Swine influenza A virus (swIAV) infection causes substantial economic loss and disease burden in humans and animals. The 2009 pandemic H1N1 (pH1N1) influenza A virus is now endemic in both populations. In this study we evaluated the efficacy of different vaccines in reducing nasal shedding in pigs following pH1N1 virus challenge. We also assessed transmission from immunized and challenged to naive, directly in-contact pigs. Pigs were immunised with either adjuvanted, whole inactivated virus (WIV) vaccines or viral vectored (ChAdOx1 and MVA) vaccines expressing either the homologous or heterologous influenza A virus hemagglutinin (HA) glycoprotein as well as an influenza viral pseudotype (S-FLU) vaccine expressing heterologous HA. Only two vaccines containing homologous HA, which also induced high hemagglutination inhibitory antibody titers, significantly reduced virus shedding in challenged animals. Nevertheless, virus transmission from challenged to naive, in-contact animals occurred in all groups, although was delayed in groups of vaccinated animals with reduced virus shedding.

IMPORTANCE This study was designed to determine whether vaccination of pigs with conventional, WIV or viral-vectored vaccines reduces pH1N1 swine influenza virus shedding following challenge and can prevent transmission to naive in-contact animals. Even when viral shedding was significantly reduced following challenge, infection was transmissible to susceptible co-housed recipients. This knowledge is important to inform disease surveillance and control strategies, and to determine the vaccine coverage required in a population, thereby defining disease moderation or herd protection. WIV or viral-vectored vaccines homologous to the challenge strain significantly reduced virus shedding from directly infected pigs, but vaccination did not completely prevent transmission to co-housed naive pigs.