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.