Study design and study population:
A cross-sectional study was conducted on patients aged ≥18 years hospitalized with AMI to the cardiology unit of National Institute of Cardiovascular Diseases in Dhaka, Bangladesh (NICVD). The patients were enrolled during the study period between 1st May 2017 to 31st October 2018 which included the 2017 annual influenza season (rainy season; between 1stMay-31st October) and the 2018 annual influenza seasons (rainy season; between 1stMay-31st October) and non-influenza season (winter; November 1, 2017 to April 30, 2018). The description of annual influenza seasons for Bangladesh based on national surveillance data is discussed in detail by Zaman et al (27). Bangladesh is the eighth-most populous country in the world, with a population exceeding 165 million and also among the most densely populated countries in the world (29). The NICVD is the largest government operated tertiary level acute care teaching hospital in Bangladesh specialized exclusively in management of cardiovascular patients travelling from all regions of the country. Project staff followed a standardized protocol to identify and recruit cases with AMI, diagnosed on electrocardiogram (ECG) findings (ST-segment elevation or depression, pathological Q waves) and ischemic symptoms (chest or arm pain, nausea or vomiting, sweating, shortness of breath), with either a change in blood level of cardiac biomarkers of myocardial necrosis (typical rise and gradual fall in Troponin or more rapid rise and fall in Creatine Kinase MB) or coronary artery intervention (30). To minimize the potential misclassification bias, after reviewing the criteria for case definition of AMI from hospital records, staff consulted with attending cardiologist to further ensure the diagnosis of AMI. Cases with previous history of cardiovascular event were eligible. Cases with self-reported history of chronic liver impairment, chronic renal impairment, malignancy, and autoimmune disorders or those not providing consent were excluded from the study.