Self-amplifying mRNA bicistronic influenza vaccines raise cross-reactive immune responses in mice and prevent infection in ferrets

Vaccines are the primary intervention against influenza. Currently licensed inactivated vaccines focus immunity on viral hemagglutinin (HA). Self-amplifying mRNA (sa-mRNA) vaccines offer an opportunity to generate immunity to multiple viral proteins, including additional neuraminidase (NA). This evaluation of a bicistronic approach for sa-mRNA vaccine development compared subgenomic promoter and internal ribosome entry site strategies and found consistent and balanced expression of both HA and NA proteins in transfected cells. In mice, sa-mRNA bicistronic A/H5N1 vaccines raised potent anti-HA and anti-NA neutralizing antibody responses and HA- or NA-specific CD4+ and CD8+ T cell responses. The addition of NA also boosted the cross-neutralizing response to heterologous A/H1N1. Similar immunogenicity results were obtained for bicistronic seasonal A/H3N2 and B/Yamagata vaccines. In ferrets, sa-mRNA bicistronic A/H1N1 vaccine fully protected lung from infection by homologous virus and showed significant reduction of viral load in upper respiratory tract, warranting further evaluation of sa-mRNA bicistronic vaccine in humans.