Clinical Research Directory

Inclusion Criteria: * Aged 18-45 years at enrolment. * Capable of providing informed consent. * Resident in Hong Kong in the coming 2 years. Exclusion Criteria: * Vaccinated against influenza in the past 24 months. * 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). * With diagnosed medical conditions related to their immune system. * Currently taking medication for any condition that impairs immune system. * 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. * 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). * Individuals who have any medical conditions not suitable to receive inactivated influenza vaccines as determined by a clinician.