Background: Hong Kong is located in subtropical regions with two seasonal peaks of influenza transmission. North Hemisphere seasonal influenza vaccine is usually available for vaccination by general public starting in October every year before the anticipated winter peak starting in December in Hong Kong, and it presumably also provides protection against the same prevailing strains during summer peak starting around July of the next year. However, influenza vaccines may have poorer efficacy and effectiveness in older adults. The investigators hypothesize that in a subtropical or tropical location with prolonged circulation of influenza viruses, including the emergence of new strains at different times of the year, twice-annual vaccination with the latest available strains could provide the best protection.