Epidemiology of influenza in Hanoi, Vietnam, from 2001 to 2003

Objective: The aim of this study was to clarify the epidemiology of laboratory-confirmed influenza in Hanoi, Vietnam. Methods: Influenza was detected by virus isolation from nasopharyngeal swabs of influenza-like-illness (ILI) patients who reported to outpatient clinics in Hanoi, Vietnam between 2001 and 2003, before the start of avian influenza A/H5N1 outbreaks. Influenza isolates were characterized by hemagglutinin inhibition test. Results: A total of 4708 nasopharyngeal swabs were collected from patients with ILI. Influenza was positive in 119 (2.5%) samples by virus isolation. Influenza circulated throughout the year, with possible two peaks in summer and winter. Influenza B viruses and A/H3N2 predominated in 2001 and 2002, respectively, and mixed circulation of A/H1N1, A/H3N2 and B were observed in 2003. The seasonality of influenza roughly matched with clinical case reports in the North Region by National Communicable Disease Surveillance in Vietnam. Conclusions: The findings of year-round and biannual peak circulation of influenza in a subtropical. area were in accordance with the results of previous studies in tropical and subtropical regions. Our observations indicated that establishment of Laboratory- based surveillance in tropical and sub-tropicat countries is important for taking actions for pandernic strategies, and links to the WHO global influenza network. (c) 2006 The British Infection Society. Published by Elsevier Ltd. All rights reserved.