Inclusion Criteria:

• Healthy subjects aged 18-40 years, mentally competent, who have signed an informed consent form after having received a detailed explanation of the study protocol;
• Able to understand and comply with all study procedures and to complete study diaries, to be contacted, and to be available for study visits.

Exclusion Criteria:

• Receipt of another investigational agent within 4 weeks prior to enrollment or before completion of the safety follow-up period in this or in another study; unwilling to refuse participation in another clinical study through the end of this study;
• Receipt of an H5N1 vaccine;
• Receipt of another vaccine within 3 weeks prior to Visit 1 or planned vaccination before Visit 5, or within 3 weeks prior to Visit 7 or before Visit 9;
• Influenza vaccination for the current season within 2 months prior to enrollment (seasonal influenza vaccination is allowed after Visit 5, but no later than 2 months before the booster dose);
• Experience of any acute disease or infection requiring systemic antibiotic or antiviral therapy (chronic antibiotic therapy for urinary tract prophylaxis is acceptable) within 7 days prior to enrollment and prior to the booster dose or fever within 3 days prior to Visit 1 and Visit 7;
• Pregnant or breastfeeding, or females of childbearing potential who refuse to use an acceptable method of birth control during the study period for at least 6 weeks following the booster dose; and, if sexually active, who have not used a reliable birth control method for at least two months prior to study entry;
• Any serious disease, such as: cancer; autoimmune disease; diabetes mellitus type I; diabetes mellitus type II; diabetes relating to genetic defects/syndromes, diseases of the exocrine pancreas or infections; advanced arteriosclerotic disease; moderate or severe chronic obstructive pulmonary disease (COPD); asthma that is greater than mild in severity and/or has exacerbations more than 2 days per week; acute or progressive hepatic disease; acute or progressive renal disease; hyperthyroidism; medically significant bleeding disorders; history of neurological disorders;
• Body mass index (BMI) ≥ 35 kg/m2 where BMI is for obese and not for high muscle mass;
• History of (or current) drug or alcohol abuse, surgery planned during the study period or any other condition that in the investigator's opinion would interfere with safety of the subject or the evaluation of study objectives or the visit schedule;
• Hypersensitivity to any component of the study vaccine;
• Known or suspected impairment/alteration of immune function;
• Members of the research staff or their relatives.