INTRODUCTION
Influenza and respiratory syncytial virus (RSV) are respiratory pathogens that cause a substantial disease burden among adults aged 65 years and above [1]. Global estimates suggest that around two-thirds of all seasonal influenza-related mortality occurs among older adults with more than 260,000 deaths annually [2]. Data from developed countries show that respiratory illnesses due to RSV among older adults may exceed one million episodes and cause around 14,000 in-hospital deaths every year [3]. Older adults with cardiopulmonary conditions are at increased risk for severe illness due to influenza and RSV infection [4, 5]. However, it is not fully established whether cardiopulmonary conditions present an increased risk for influenza and RSV infections, and there are little data on the burden of infection from these pathogens among persons with cardiopulmonary conditions in middle-income countries.
Thailand is an upper-middle-income country in tropical Southeast Asia, where recognition of influenza and RSV disease burden is growing. Since 2008, adults aged ≥65 years and those with chronic diseases have been recommended for annual influenza vaccination by the Thai Ministry of Public Health (MoPH) [6]. Nonetheless, influenza vaccine uptake among older adults is still less than 20% (Bureau of General Communicable Diseases, Thailand MoPH). Studies among hospitalized patients suggest that the prevalence of RSV-associated respiratory illnesses in Thailand is around 2-9% among older adults [7-11]. However, reliable estimates of the infection rates in the community remain limited. Using data from a longitudinal prospective cohort study of influenza vaccine effectiveness among rural community-dwelling Thai adults aged ≥65 years [12], we compared the incidence of influenza and RSV in those with and without cardiopulmonary conditions.