Intranasal hemagglutinin protein boosters induce protective mucosal immunity against influenza A viruses in mice

Licensed parenteral influenza vaccines induce systemic antibody responses and alleviate disease severity but do not efficiently induce local mucosal immune responses. Here, we describe an intranasal booster strategy with unadjuvanted recombinant hemagglutinin (HA) following initial messenger RNA-lipid nanoparticle (mRNA-LNP) vaccination, Prime and HA. This regimen establishes highly protective HA-specific mucosal immune memory responses in the respiratory tract. Intranasal HA boosters resulted in significantly reduced viral replication compared to parenteral mRNA-LNP boosters in both young and old mice. Correlation analysis revealed that slightly increased levels of nasal Immunoglobulin A (IgA) are significantly associated with a reduced viral burden in the upper respiratory tract. Intranasal boosting with bivalent H1 HA induced mucosal immunity against vaccine-matched and mismatched heterologous influenza viruses. Additionally, a heterosubtypic intranasal H5 HA booster elicited H5-reactive mucosal humoral responses in H1-surviving mice. Our work illustrates the potential of a nasal HA protein booster as an adjuvant-free strategy to prevent infection and disease from influenza A viruses.