A phase I study of the safety, reactogenicity and immunogenicity of two quadrivalent seasonal influenza vaccines (Fluzone® or Flublok®) with or without one of two adjuvants (AF03 or Advax-CpG55.2) in healthy adults 18-45 years of age.

Seasonal influenza continues to cause significant morbidity and mortality, particularly for the elderly and immunocompromised. Current licensed influenza vaccines provide only partial protection even for immunocompetent hosts. Vaccine adjuvants can improve the magnitude and breadth of immune responses and there is considerable interest in identifying new adjuvants that can improve immune responses to seasonal influenza vaccines. This phase I, randomized, double-blind trial evaluated the safety and immunogenicity of one dose of 2018/2019 quadrivalent influenza vaccine (either Fluzone® or Flublok®) administered intramuscularly with or without one of two adjuvants (AF03 or Advax-CpG55.2). A total of 241 healthy adults aged 18-45 years were enrolled and randomized to 1 of 6 groups. Groups 1-3 received one dose of Fluzone® QIV 2018/2019 administered alone or with AF03 or Advax-CpG55.2 and Groups 4-6 received one dose of Flublok® QIV 2018/2019 alone or with one of these two adjuvants. All participants received Fluzone® or Flublok® QIV 2019/2020 ninety days later. Primary objectives were to evaluate safety and reactogenicity along with changes in hemagglutinin inhibition (HAI), neuraminidase inhibition (NAI) and neutralizing antibodies to 2018/2019 seasonal influenza antigens, comparing Day 1 and Day 29 titers. Secondary objectives evaluated the impact of adjuvants on immune responses after subsequent doses of unadjuvanted seasonal influenza vaccine and immunologic responses to heterologous influenza H1 and H3 antigens. Overall, the adjuvanted vaccines were safe and generated robust immune responses against both homologous and heterologous strains. Similar responses were seen across all six study arms. Both adjuvants were associated with qualitatively improved immune responses against some strains at varying timepoints, but results were inconsistent. There were no substantial differences in safety or reactogenicity identified between the study groups and all vaccine formulations were well tolerated. In this highly immunologically-experienced cohort, neither AF03 nor Advax-CpG55.2 demonstrated notable benefit when added to the seasonal influenza vaccine. (ClinicalTrials.gov ID# NCT03945825).