An RNA replicon vaccine encoding HA and NA prevents shedding of antigen-drifted 2009 pandemic H1N1 influenza virus in the pig model

Seasonal influenza viruses escape the human immune response by antigenic drift, i.e. the positive selection of point mutations that prevent the binding of inhibitory antibodies to the influenza antigens HA and NA. The efficacy of seasonal influenza vaccines can be less than 50% if the selected influenza vaccine strain does not match the antigenic characteristics of the circulating seasonal influenza virus. In this study, we used the porcine model to evaluate the efficacy of an RNA replicon vaccine encoding the HA and NA antigens of A/Hamburg/4/2009 (H1N1) (H1N1HH4/09) in inducing cross-reactive immunity. We found that a single intramuscular immunization with this vaccine elicited high levels of antibodies with H1N1HH4/09-neutralizing activity and potent N1-sialidase inhibition. A second immunization with the same H1/N1 RNA replicon particles or with a live-attenuated influenza vaccine (LAIV) based on a modified H1N1HH4/09 virus boosted the inhibitory activity of the immune sera against the antigen-drifted A/Victoria/2570/2019 (H1N1) (H1N1Vic/19) strain. Interestingly, vaccination elicited N1-specific antibodies that also inhibited the activity of avian N1 sialidase and potently inhibited the replication of A/cattle/Texas/063224-24-1/2024 (H5N1) (H5N1Tex/24) in vitro. When challenged nasally with a H1N1HH4/09 /H1N1Vic/19 6:2 reassortant virus encoding the HA and NA antigens of H1N1Vic/19, immunized pigs did not shed infectious virus while the control animals did, suggesting that homologous prime/boost vaccination with H1/N1 replicon particles can block virus replication in the upper respiratory tract as efficiently as the heterologous RNA replicon prime/LAIV boost immunization regimen. In conclusion, RNA replicons encoding both HA and NA either used alone or in combination with LAIV mediate protection against antigen-drifted influenza viruses and reduce the risk of vaccination breakthroughs due to antigen mismatch. Furthermore, this vaccine may also limit the infection by zoonotic H5N1 viruses.