Nasal RNA-scaffold-protein vaccine protects mice from human H5N1 clade 2.3.4.4b virus lethal infection and safeguards against vaccine-unmatched viruses.

BACKGROUND: Recent cases of H5N1 avian influenza spilling over into human populations have raised concerns once again about the unpredictable emergence of zoonotic viral diseases. This phenomenon highlights the persistent threat posed by viruses that jump from animals to humans, underscoring the need for preparedness for antigens that humans have not encountered. Nasal vaccines, which can effectively block transmission at the site of viral entry, represent a promising strategy and should be prioritised for stockpiling to halt zoonotic spillover. METHODS: Here, we immunised a preclinical animal model intranasally with the RNA-Scaffold-Protein Vaccine (RSPVac) and evaluated mucosal immunity and protection against avian-origin and pandemic human influenza viruses. FINDINGS: Nasal immunisation of strain-specific H5N1 and H1N1 RSPVac induced strong antibody responses in serum and mucosa, accompanied by mucosal antigen-specific T cells. We examined immune cells during early vaccination and found that RSPVac induced a strong but transient cytokine expression. Neutrophils comprising both regulatory and effector subsets were observed, indicating a controlled activation. Vaccinated animals were completely protected against vaccine-matched avian virus challenges without pathological signs. Protection was also observed against vaccine-unmatched H1N1 and H7N9 challenges, with a display of trained immune cell profiles. Finally, mutant RSPVac demonstrated an impaired vaccination outcome, suggesting a complex activation mechanism. INTERPRETATION: These findings demonstrate that RSPVac elicits potent mucosal immunity and safeguards against vaccine-unmatched infections, underscoring its potential as a non-infectious mucosal vaccine to prevent zoonotic virus transmission. FUNDING: This study is supported by the Health@InnoHK initiative of the Innovation and Technology Commission of the Hong Kong Special Administrative Region Government.