A Study to Evaluate Safety and Immunogenicity of One or Two Booster Vaccinations With H5N6 Influenza Vaccine in Adults Primed With H5N1 Influenza Vaccine or Unprimed

Inclusion Criteria: * Subjects who received 2 doses of aH5N1c vaccine or placebo in and completed the parent study V89\_18 in the \<65 years of age cohort. * Individuals who can comply with study procedures including follow-up. Exclusion Criteria: * Females of childbearing potential who are pregnant, lactating, or who have not adhered to a specified set of contraceptive methods from at least 30 days prior to study entry and who do not plan to do so until at least 30 days after the last study vaccination. * Progressive, unstable or uncontrolled clinical conditions. * Hypersensitivity, including allergy, to any component of vaccines, medicinal products or medical equipment whose use is foreseen in this study. * Clinical conditions representing a contraindication to intramuscular vaccination and blood draws. * Abnormal function of the immune system. * History of any medical condition considered an adverse event of special interest (AESI). * Received immunoglobulins with immunomodulating effects or any blood products within 180 days prior to informed consent. * Subjects, who received an influenza H5 vaccine other than in the V89\_18 parent study or have a history of H5 influenza infection prior to enrollment. * Received an investigational or non-registered medicinal product within 30 days prior to informed consent. * Individuals who received any other vaccines \[except corona virus disease 2019 (COVID-19) vaccines\] within 14 days (for inactivated vaccines) or 28 days (for liv…