DISCUSSION
Among a two-year prospective cohort study of >3,000 community dwelling adults in Thailand, around 10% of participants had underlying cardiopulmonary conditions and cardiopulmonary conditions conferred increased rates of all-cause ARI, influenza A and RSV infections. These findings provide additional support for influenza vaccination recommendations for persons with underlying cardiopulmonary conditions and add to the limited body of evidence about influenza and RSV disease burden among older adults in middle-income countries.
The seasonality of influenza and RSV was similar to the pattern identified in past studies in which the infections peaked during the rainy months of June to October [10, 17, 18]. Incidences of influenza and RSV are higher in our study than previous studies [7, 8, 10]. This may be partly due to a more sensitive case definition of ARI. Since many older adults with influenza and RSV can present without fever, we used a definition of ARI without any body temperature criteria [12]. This might have resulted in the inclusion of more illness episodes with laboratory confirmation than in the past studies which required either fever or hypothermia (<35.5 °C) to trigger swab collection. Moreover, previous studies primarily assessed hospitalized patients and may not have fully accounted for non-medically attended cases in the community, even after adjustments in their analyses [7, 8]. In our study, more than 80% of the respiratory specimens were nasal self-swabs collected by the participants at their homes and only a small proportion of ARI were severe enough to warrant hospitalization, signifying a majority of non-medically attended cases in the community.
Cardiopulmonary conditions are recognized risk factors for severe outcomes of influenza and RSV infections, especially among older adults [4, 5]. Prior studies have demonstrated that a disproportionate number of hospitalizations for influenza and RSV occur in patients with cardiopulmonary conditions, particularly COPD [9, 10, 18]. Our findings suggest that these conditions may themselves be predisposing risk factors for acquisition of influenza and RSV infections in the community, possibly by affecting immunoregulatory functions in older adults [19]. Our findings are consistent with evidence from Western countries that documents higher detection of RSV among older adults with underlying cardiopulmonary diseases compared to those without these conditions [19, 20].
There are some limitations to our study. First, we ascertained cardiopulmonary conditions by participant report alone which may be susceptible to some recall and measurement bias. Second, we did not collect data about exposure to children, household crowding, socioeconomic status, and baseline influenza and RSV antibody titers which may be confounders in the relationship between cardiopulmonary conditions and acquisition/transmission of influenza virus and RSV infection [19]. Third, a previous analysis comparing participant-collected nasal swabs to nurse-collected nasopharyngeal swabs among older adults in Thailand found that participant-collected nasal swabs were only 78% sensitive for detection of influenza viruses [14]. Therefore, our calculation of influenza and RSV incidence may be underestimated. Nevertheless, the strengths of this research lie in the population-based multiple-year prospective cohort study design with a large sample size, use of a broad case definition of ARI, capture of non-medically attended ARI, active weekly follow-up of participants, and laboratory-confirmation of the infection outcomes.
In conclusion, incidences of influenza and RSV were high among community-dwelling older Thai adults aged > 65 years in this study, and cardiopulmonary conditions conferred additional risk for acquisition of influenza and RSV infections. Despite efforts to enhance influenza vaccine uptake among this cohort as part of the study design, only half of participants received influenza vaccines during each of the study years. More efforts are needed to ensure adults aged> 65 years, especially those with cardiopulmonary conditions, have access to influenza vaccines and other prevention measures as effective vaccines against RSV are not currently available.