Immunogenicity, Tolerability and Safety of One or Two Doses of an Adjuvanted Swine-origin A H1N1 Influenza Vaccine in Healthy Subjects 18 or More Years of Age Both Vaccinated and Not Yet Vaccinated Against Seasonal Influenza 2009/10

Inclusion Criteria: 1. For study Groups A and B : subjects aged 18 years and above on the day of enrollment, who took part to the seasonal trials for the 2009/10 vaccine registration; For study Groups C to E: subjects aged 18 years and above on the day of enrollment, not yet exposed to the current seasonal formulation (2009/10 for NH) of the influenza vaccines; 2. Individuals in good health as determined by medical history, physical examination and clinical judgment of the investigator; 3. Documented consent obtained after the nature of the study has been explained according to local regulatory requirements; 4. Individuals are able to comply with all study procedures and are available for all clinic visits scheduled in the study. 5. Willingness to allow for serum samples to be stored beyond the study period, for potential additional future testing to better characterize immune response. Exclusion Criteria: 1. Individuals who are not able to comprehend and to follow all required study procedures; individuals with behavioral or cognitive impairment or psychiatric disease that, in the opinion of the investigator, may interfere with the subject's ability to participate in the study; 2. Individuals with history or any illness that, in the opinion of the investigator, might interfere with the results of the study or pose additional risk to the subjects due to participation in the study; 3. Individuals who have had received an influenza vaccine for the 2009/10 NH influenza season (for subjects to be included in Groups C to E only) prior to Visit 1; 4. Individuals who have had documented confirmed or suspected influenza disease within 6 months prior to Day 1. 5. 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; 6. Individuals who received any other vaccines within 4 weeks prior to enrollment in this study and individuals who have ever received blood, blood products and/or plasma derivatives in the past 12 weeks; 7. Individuals with axillary temperature ≥ 38.0°C within 3 days of study vaccination; 8. Known or suspected impairment/alteration of immune function, for example resulting from: * receipt of immunosuppressive therapy, * cancer chemotherapy, * receipt of immunostimulants within 60 days prior to Visit 1, * receipt of parenteral immunoglobulin preparation, blood products, and/or plasma derivatives within 3 months prior to Visit 1 or planned during the full length of the study, * known HIV infection or HIV-related disease. 9. Surgery planned during the study period; 10. If female, of childbearing potential, and has not used any of the "acceptable contraceptive methods" for at least 2 months prior to study entry; 11. Females who are pregnant or nursing (breastfeeding) mothers, or females of childbearing potential who are sexually active and have not used or do not plan to use acceptable birth control measures during the first 3 weeks after vaccination. 12. Members of the research staff or their relatives (research staff are individuals with direct or indirect contact with trial subjects, or study site personnel who have access to any study documents containing subject information, including: e.g. receptionists, persons scheduling appointments or making screening calls, regulatory specialists, laboratory technicians); hospital personnel and health care providers not involved in the study can be enrolled.