In the majority of recruiting sites, the criteria for further household follow-up will be a positive result for influenza A or B using the QuickVue rapid diagnostic test on a nose and throat swab (Quidel Corp, San Diego, CA) which has a reported sensitivity of 79% and specificity of 92% for influenza A or B [23] . If the QuickVue test is positive and informed consent is obtained, the index case and their household will be further followed up with home visits as described above. We will calculate the sensitivity and specificity of the rapid diagnostic test in our study setting, using viral culture or PCR of a nose and throat swab as the gold standard.