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.