The Immune Response to Influenza Vaccinations in Elderly Individuals

Inclusion Criteria: 1. Aged ≥ 65 years and \<90 years on the day of inclusion for the elderly group, and ≥ 21-\<40 for the adult group. 2. Informed consent form has been signed and dated. 3. Able to attend all scheduled visits and to comply with all trial procedures. 4. Living at home. 5. Able to walk without personal assistance and no other physical limitations that can limit participation. 6. Recruited from the cohort in Singapore Longitudinal Ageing Study-II (SLAS-II), as well as from sites in West Jurong and from Outpatient clinics at National University Hospital (NUH). Exclusion Criteria: 1. Participation at the time of study enrollment (or in the 4 weeks preceding trial vaccination) or planned participation during the present trial period in another clinical trial investigating vaccine, drug, medical device, or medical procedure. 2. Receipt of any vaccine in the 4 weeks preceding the trial vaccination or planned receipt of any vaccine in the 4 weeks following trial vaccination, including influenza vaccination. However, 23-valent pneumococcal vaccine and tetanus vaccine will be allowed during this time window. 3. Receipt of an influenza vaccine within the 6 months preceding the trial vaccination or planned influenza vaccination during the trial. 4. Known or suspected congenital or acquired immunodeficiency or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy, within the preceding 6 months; or long-term systemic corticosteroid therapy (prednisone or equivalent for more than 2 consecutive weeks within the past 3 months). 5. Systemic hypersensitivity to eggs, chicken proteins, or any of the vaccine components or a history of a life-threatening reaction to Vaxigrip® or to a vaccine containing any of the same substances. 6. Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalized involuntarily. 7. Current alcohol or drug addiction. 8. Chronic illness that, in the opinion of the investigator, is at a stage where it might interfere with trial conduct or completion. 9. Identified as an Investigator or employee of the Investigator or study centre with direct involvement in the proposed study, or identified as an immediate family member (i.e. parent, spouse) of the Investigator or employee with direct involvement in the proposed study. 10. Severe audio-visual impairment. 11. Dementia, severe cognitive impairment (MMSE \<18), major depression or other psychotic disorders. 12. Progressive, degenerative neurologic disease: e.g. Alzheimer's disease. 13. Rapidly progressive or terminal illness under palliative care with life expectancy less than 12 months. 14. Primary severely muscle/joint disorders resulting in physical disability interfering with the physical performance tests needed for the study. 15. Hospital admission in the past 6 weeks. 16. (Relevant for the healthy young adults subjects group). Subject is pregnant (or positive urine pregnancy test), or lactating, or of childbearing potential (to be considered of non-childbearing potential, a female must be postmenopausal for at least 1 year, surgically sterile, or using an effective method of contraception or abstinence from at least 4 weeks prior to vaccination and until at least 4 weeks after vaccination). 17. Self-reported thrombocytopenia, contraindicating intramuscular vaccination 18. Bleeding disorder, or receipt of anticoagulants in the 3 weeks preceding vaccination, contraindicating intramuscular vaccination upon investigator's judgement.