The 8-week pilot study will take place from January to April 2007. The exact starting and stopping dates of patient recruitment will not be fixed in advance. Recruitment will begin after the start of the annual influenza peak season (typically Feb/Mar) has been confirmed by the Department of Microbiology, HKU, and will continue until the prespecified sample size is reached. The 39-week main study will take place from January to September 2008.