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.