In the main study, clinical influenza is defined as the presence of at least two of the following symptoms: feverishness (we will strongly encourage household members to use the supplied thermometer to assess whether a fever is ≥38°C); cough; headache; sore throat; pain in muscles or joints (following [24] ). In the pilot study (as per the initial protocol), clinical influenza is defined as the presence of feverishness, or at least two of the following symptoms: cough; sore throat; nasal congestion, rhinorrhoea, or sneezing; fatigue; headache; stiffness; myalgias.