Rare Adenoviral Vectored Vaccines for Emerging Influenza Threats

Background: Emerging and re-emerging infectious diseases pose a growing global threat, underscoring the need for vaccine platforms enabling rapid deployment and durable, broad protection. Although mRNA and viral vector vaccines transformed pandemic responses, limitations include waning immunity, restricted antigenic breadth, pre-existing vector immunity, and rare vector-associated adverse events. Here, we evaluate recombinant adenovirus serotypes 4, 28, and 48 expressing a centralized H5 influenza hemagglutinin consensus antigen as next-generation vectors for broad, single-dose protection against diverse H5 influenza clades. Methods: An H5 centralized consensus vaccine was designed using human sequences from the 2003–2006 bird flu outbreak and cloned into four low-prevalence recombinant adenoviruses. Mice received a single intramuscular or intranasal vaccination and were challenged two weeks later with high doses of diverse, lethal, reverse-genetics-derived H5N1 viruses. Immune correlates, survival, and protection from challenge-associated weight loss were assessed. Results: Vaccinated mice were largely protected from lethal challenge against strains spanning twenty years of influenza evolution. Both vaccination routes conferred complete protection against antigenically related strains, and strong protection against more divergent clade 2.3.4.4b strains. Protection occurred before neutralizing antibodies could be detected, although strong H5 antibody responses were induced by all vaccines. Intramuscular vaccination generated strong serum IgG and antigen-specific T cell responses, while intranasal vaccination promoted robust antigen-specific mucosal IgA production. Conclusions: Adenovirus vectors expressing a centralized H5 consensus hemagglutinin elicit rapid, broad protection against diverse H5 influenza viruses following a single intramuscular or intranasal immunization. These results support the continued development of low-seroprevalence adenoviral vectors as a next-generation influenza vaccine platform for rapidly emerging influenza viruses.