Inclusion criteria for index cases are as follows: (1) a Hong Kong resident; (2) reporting ILI symptoms including at least two of fever (recorded fever ≥38°C), cough; nasal congestion; sore throat; headache; runny nose and pains in muscles or joints [24] (3) onset of symptoms within the preceding 48 hours. If these conditions are satisfied, the subject will be approached to determine household eligibility to enrol in the study as below prior to further follow-up as described in 4(d) and 4(f).