The Dynamics of the Immune Responses to Repeat Influenza Vaccination Exposures (DRIVE III) Study

Inclusion Criteria: 1. Aged 22-50 years at enrolment. 2. A. Participated in the DRIVE I or DRIVE II studies (for DRIVE IIIa). OR B. Did not participate in the DRIVE I or DRIVE II study (for DRIVE IIIb) and has not received influenza vaccination in the prior two years. 3. Capable of providing informed consent. 4. Resident in Hong Kong in the coming 2 years. Exclusion Criteria: 1. Included in one of the priority groups to receive influenza vaccination in Hong Kong (priority groups include pregnant women, long-stay residents of institutions for persons with disability, persons with chronic medical problems (chronic cardiovascular, lung, metabolic or kidney diseases, obesity (body mass index 30 or above) and chronic neurological condition ), healthcare workers or persons working in poultry, pig farming or pig slaughtering industry). 2. With diagnosed medical conditions related to their immune system. 3. Currently taking medication for any condition that impairs immune system. 4. Individuals who report medical conditions not suitable to receive inactivated influenza vaccines, such as: * Severe allergic reaction (e.g., anaphylaxis) after previous dose of any influenza vaccine; or to a vaccine component; * Moderate or severe acute illness with or without fever after any previous influenza vaccination; or * A history of Guillain-Barré syndrome (GBS) within 6 weeks of previous influenza vaccination. 5. Individuals who report medical conditions not suitable to receive live attenuated vaccines, such as: * having asthma; * having close contact with severely immunosuppressed persons who require a protected environment; or * having immunosuppressive treatment (e.g. high-dose steroid, anti-cancer drugs and radiotherapy). 6. Individuals who report medical conditions not suitable to receive intramuscular injection, such as * Bleeding disorders * Habitually taking anticoagulants (with the exception of antiplatelets such as aspirin). 7. Individuals who have any medical conditions not suitable to receive inactivated or live attenuated influenza vaccines as determined by a clinician.