Study design
Details of this study have been described previously [12]. In brief, a prospective cohort of 3500 adults aged ≥65 years was recruited by systematic random sampling in two districts of Nakhon Phanom province, Thailand from May 2015 and followed for two years until May 2017. Prior to the start of the study, a targeted campaign was organized in the study sites by the Thai MoPH to enhance influenza vaccine uptake among older adults through mobile clinics and increased vaccine supply [13]. Individuals were not eligible to participate in the cohort if they resided in a long term care facility, did not reside in the community for at least a year, were unable to communicate with study staff, were unable/unwilling to provide nasal self-swabs, or had an acute medical condition or tumor in or near the nose that would preclude nasal swab collection. Trained health volunteers collected baseline data and followed the participants with weekly telephone surveillance for acute respiratory illness (ARI). Participants were asked to self-collect anterior nasal swabs with each ARI episode as previously validated [14]. For participants who visited health centers/hospitals for their illnesses, nasopharyngeal swabs were collected by research nurses on site. The number of hospital visits of the participants (for ARI or other reasons) during the study years were obtained from the district and provincial hospital records.