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

This is a Phase 2, randomized, multi-center study in approximately 300 adults who received 2 doses of aH5N1c or placebo in and completed the parent study V89_18 in the \<65 years of age cohort. The study investigates whether two priming doses of MF59-adjuvanted H5N1 cell culture-derived vaccine (aH5N1c) followed by one or two booster vaccinations with a MF59-adjuvanted H5N6 cell culture derived vaccine (aH5N6c) 3 weeks apart elicit immune responses to the antigens used for priming (H5N1) and boosting (H5N6) after first and second heterologous booster vaccination.

Eligible subjects, who received 2 doses of aH5N1c in the parent study V89_18 are randomized in a 1:1 ratio to receive either two aH5N6c vaccinations, 3 weeks apart (group 1) or an aH5N6c vaccination on Day 1 and saline placebo on Day 22 (group 2). Eligible subjects, who received placebo in the parent study will receive two aH5N6c vaccinations, 3 weeks apart (group 3). After the second vaccine administration, subjects are monitored for approximately 6 months for safety and antibody persistence. The total study duration will be approximately 7 months per subject.


Eligibility Criteria:
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 live vaccines) prior to enrollment in this study or who are planning to receive any vaccine within 28 days from the study vaccination.
* Receipt of any COVID-19 vaccine within 7 days prior to enrollment or plan to receive any COVID-19 vaccine within 7 days from study vaccination.
* Acute (severe) febrile illness.
* A known history of Guillain-Barre Syndrome or other demyelinating diseases such as encephalomyelitis and transverse myelitis.