Strengths:
Influenza-associated acute respiratory illness (ARI) accounts for millions of severe cases and more than half a million deaths worldwide every year(1, 2). Adults with comorbid illness and those aged more than 65 years are at higher risk of severe influenza outcomes(1) such as intensive care unit admissions (3). The 2009 pandemic H1N1 influenza is thought to have contributed to more than 80,000 additional cardiovascular deaths globally indicating an unrecognized burden of severe influenza(4). Furthermore, adverse cardiovascular incidents such as acute myocardial infarction (AMI) and stroke contribute to almost one-third of all deaths worldwide, and the majority of the global burden of AMI is currently observed in low- and middle-income countries, such as Bangladesh, where it is on a yearly rise (5-7).
An AMI may be precipitated from a vulnerable atherosclerotic plaque rupture after short term exposure of certain triggering factors such as acute respiratory infection(8). Increase in incidence of acute cardiovascular events such as AMI and stroke during winter months has been attributed to ARI, along with other determinants over the past years(9). Temporal variations in acute cardiovascular events were reported as early as 1937 (10) and several time-series analyses showed correlations of seasonal peaks of influenza-associated mortality with cardiovascular deaths (11-15). Meta-analysis of multiple case-control studies revealed that AMI cases were associated with greater incidence rates of recent acute respiratory illness, influenza-like illness, and laboratory-confirmed influenza compared to control groups (pooled OR 2.01; 95% CI 1.47 to 2.76) (16), which suggests that recent ARI and influenza may significantly contribute to the occurrence of AMI events. Furthermore, self-controlled case-series investigations have reported a heightened risk of AMI in the immediate aftermath of acute respiratory illness and laboratory-confirmed influenza (17-20). Influenza and ARI have also been known to exacerbate myocardial injury, as indicated by troponin levels (21). Although influenza-associated AMI may go unnoticed, it can be prevented through influenza-specific preventive measures, such as influenza vaccination, which has been proven effective in various large-scale randomized controlled trials (RCTs) (22) as well as several smaller RCTs (23-25).
Like other low- to middle-income countries, an inadequately investigated double burden of communicable illnesses such as ARIs and non-communicable diseases such as adverse acute cardiovascular events like AMI prevails in Bangladesh (26). National surveillance data confirms annual seasonal influenza prevalent among hospitalized patients throughout Bangladesh with unimodal peak during rainy season and may be associated with deaths among older adults and elderly (27, 28). Despite the existing burden of incidences of AMI, as well as the concomitant evidence of circulating influenza among the population there is lack of data and awareness on the prevalence of influenza as well as recent ARI exclusively among AMI patients in Bangladesh, which incited the current investigation. Therefore, the primary research question was about the prevalence of recent ARI and laboratory evidence of influenza among patients with AMI in Bangladesh. This information is crucial for understanding the extent of the impact of these illnesses on the cardiovascular health of the population and for developing targeted interventions. To address the gap, the current exploratory study aimed to estimate the prevalence of recent respiratory illness and laboratory-confirmed influenza among patients hospitalized with AMI. A secondary aim was to explore the association of recent respiratory illness with severity of infarction. As an initiative, we opted for a cross-sectional design without a control group for its time efficiency, simplicity and ability to include a larger population.