Specimen collection and laboratory analysis:
Trained staff collected nasopharyngeal and oropharyngeal swabs at recruitment within 72 hours of date and time of AMI onset. Specimen collection within the aforementioned timeframe was conducted in order to maximize the sensitivity of the study to detect influenza RNA in the swabs prior to the cessation of viral shedding. The specimens were transported daily in Viral Transport Medium (VTM) at 2- 8o C to virology laboratory of International Centre for Diarrheal Disease Research, Bangladesh (icddr,b), in Dhaka. The specimens were aliquoted in a BSL-2 (bio-safety level-2) safety cabinet and were stored in freezers at or below -70 o C until analysis. Viral RNA was extracted from the swab specimen, and Real time Reverse Transcription Polymerase Chain Reaction (qRT-PCR) was performed using primers and probes specific for influenza A and B viruses provided by Influenza Division at CDC. Hemagglutinin subtyping of type A and B viruses was performed to detect subtypes, A/H3, A/H1N1pdm09, B/Victoria, B/Yamagata. The laboratory used validated protocols (31) and followed standard quality control procedures.