Elicitation of protective immune responses against influenza a viruses in elderly ferrets by adjuvanted recombinant universal influenza hemagglutinin vaccines

Influenza viruses remain a significant threat to both animals and public health. Each season influenza virus infection causes significant morbidity and mortality, especially in the elderly population. Annual vaccination boosts antibody titers against circulating influenza strains in elderly individuals, albeit the antibody responses are not as robust as those in younger adults. In addition, the elderly have a rapid decline in antibody titers over a 6-month period following an influenza virus immunization with antibody titers often returning to baseline at the beginning of each influenza season. Thus, there is a need for next-generation influenza vaccines that induce broadly and long-lasting protective immunity against influenza viruses in high-risk, elderly populations. In this study, Computationally Optimized Broadly Reactive Antigen (COBRA)-based H1 and H3 hemagglutinin (HA) vaccines were tested in combination with toll-like receptor (TLR) agonists as adjuvants in elderly ferrets with immune memory to historical influenza viruses to assess the breadth of protective antibody responses. These COBRA vaccines were mixed with either TRAC478, which is a combination of two TLR agonists, INI-2002 (a synthetic TLR4 agonist) and INI-4001 (a synthetic TLR7/8 agonist), or SAS, which is a mixture of INI-2002 and a semi-synthetic saponin. Elderly ferrets vaccinated with H1/H3 COBRA HA proteins plus TRAC478, or SAS had antibodies with hemagglutination-inhibition (HAI) activity against a panel of H1N1 and H3N2 influenza viruses that were significantly higher than animals vaccinated with unadjuvanted HA vaccine only. These elderly ferrets were protected following challenge with influenza virus with little to no detectable signs of disease or weight loss. In addition, there were reduced viral titers detected in nasal washes compared to elderly ferrets vaccinated with unadjuvanted HA proteins. Overall, both TRAC478 and SAS were effective adjuvants to enhance protective antibodies in elderly ferrets compared to animals that were vaccinated with only COBRA HA protein.