Influenza 5xM2e mRNA lipid nanoparticle vaccine confers broad immunity and significantly enhances the efficacy of inactivated split vaccination when coadministered

Current influenza vaccines are not effective in conferring protection against antigenic variants and pandemics. To improve cross-protection of influenza vaccination, we developed a 5xM2e messenger RNA (mRNA) vaccine encoding the tandem repeat conserved ectodomain (M2e) of ion channel protein M2 derived from human, swine, and avian influenza A viruses. The lipid nanoparticle (LNP)-encapsulated 5xM2e mRNA vaccine was immunogenic, eliciting high levels of M2e-specific IgG antibodies, IFN-γ+ T cells, T follicular helper cells, germinal center phenotypic B cells, and plasma cells. The mice with 5xM2e mRNA vaccination were broadly protected against lethal infection regardless of hemagglutinin (H1, H3, H5) subtypes by preventing severe weight loss. Injection of 5xM2e mRNA LNP vaccine induced acute innate responses recruiting monocytes, macrophages, and diverse subsets of dendritic cells. A single dose of combined 5xM2e mRNA LNP and split vaccines resulted in significantly enhanced and sustainable IgG antibody responses to viral antigens and protection against homologous and heterologous viruses. This study provides a new strategy of combined mRNA and seasonal vaccination, significantly enhancing vaccine protective efficacy.