The criteria for further follow-up of the index subject and their household were described in 4(d) above. In the majority of cases, the criteria for further follow-up will be a positive result on a rapid diagnostic case. In some recruiting clinics the criteria will be presentation with at least two of the following symptoms: fever≥37.8°C (≥38°C in the pilot study); cough; headache; sore throat; pain in muscles or joints. However we will only include households in the final analyses if the index subject is laboratory confirmed to have influenza infection. This laboratory confirmation will require a positive result for influenza A or B by viral culture or standard PCR of a nose and throat swab collected from the index case at the recruitment site, and/or during the first home visit.