All participants in this study were specially trained nurses, who provided nursing care and treatment for patients in their own homes (also known as Community Nursing Service) in Hong Kong. The responsibility of these community nurses is to provide nursing care and health education to patients through home visits. CNS nurses are employed by Hospital Authority in Hong Kong and provide continuity of care for patients who have been discharged from hospitals such that patients can recover in their own homes. Community nurses were chosen because of their frequent contacts with patients in their homes which is likely to increase their risk for exposure to influenza. We have only included CNS nurses who provide medical services in the study. The rest of the CNS nurses (around 100 nurses) provide psychiatric services in the community.