We recruited healthcare workers only in public hospitals in this study because 94% of secondary and tertiary healthcare services in Hong Kong are provided by these hospitals. Pandemic influenza patients will be primarily treated in these hospitals-as occurred in the outbreak of SARS (severe acute respiratory syndrome), when public healthcare workers were at the highest risk for contracting infection. All department heads (42 departments with 8508 doctors and nurses) of emergency medicine, internal medicine, and paediatrics units under the Hong Kong Hospital Authority were contacted through emails, invitation letters, or telephone calls to obtain approval to send the questionnaires to their staff. These departments were chosen as their staff are at the highest risk of exposure to patients with influenza. We also sent invitation letters to the physiotherapy and occupational therapy departments under the Hospital Authority.