The purpose of this Phase II follow-up study (to study 810501) is to assess the immunogenicity and safety of a booster vaccination with a non-adjuvanted H5N1 influenza vaccine administered 12 to 17 months after a two-dose regimen of different doses of an adjuvanted or non-adjuvanted H5N1 influenza vaccine in a healthy young adult population. A data safety monitoring board will review the safety data after the booster vaccination. The H5N1 influenza antibody persistence 180 days after the booster vaccination will also be assessed.

Inclusion Criteria:

Subjects who participated in Study 810501 will be eligible for participation in the study if they:

• Completed the Day 42 visit in study 810501
• Have an understanding of the study, agree to its provisions, and give written informed consent prior to study entry
• Are clinically healthy, as determined by the Investigator's clinical judgment through collection of medical history and performance of a physical examination
• Agree to keep a daily record of symptoms for the duration of the study
• If female and capable of bearing children, have a negative urine pregnancy test result at study entry and agree to employ adequate birth control measures for the duration of the study

Exclusion Criteria:

• Have a history of vaccination with an H5N1 influenza virus since the second vaccination in study 810501
• Have had an allergic reaction to one of the components of the vaccine since the second vaccination in Study 810501
• Have been diagnosed with a significant neurological, cardiovascular, pulmonary (including asthma), hepatic, rheumatic, autoimmune, hematological, renal or metabolic disorder since the second vaccination in Study 810501
• Are unable to lead an independent life as a result of either physical or mental handicap
• Suffer from any kind of immunodeficiency since the second vaccination in Study 810501
• Suffer from a disease or were undergoing a treatment within 30 days prior to the scheduled booster vaccination or are currently undergoing a form of treatment that can be expected to influence immune response. Such treatment includes, but is not limited to, systemic or high dose inhaled (>800µg/day of beclomethasone dipropionate or equivalent) corticosteroids, radiation treatment or other immunosuppressive or cytotoxic drugs
• Have had severe allergic reactions or anaphylaxis since the second vaccination in Study 810501
• Have a rash, dermatologic condition or tattoos which may interfere with injection site reaction rating
• Have undergone systemic corticoid therapy within 30 days prior to study entry
• Have a functional or surgical asplenia
• Have a known or suspected problem with alcohol or drug abuse
• Were administered an investigational drug within six weeks prior to study entry or are concurrently participating in a clinical study that includes the administration of an investigational product
• Are a member of the team conducting this study or are in a dependent relationship with the study investigator. Dependent relationships include close relatives (i.e., children, partner/spouse, siblings, parents) as well as employees of the investigator If female, are pregnant or lactating