title: Epidemiology of influenza in Hanoi, Vietnam, from 2001 to 2003.
abstract: 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-tropical countries is important for taking actions for pandemic strategies, and links to the WHO global influenza network.
authors.affiliation: National Institute of Hygiene and Epidemiology, 1 Yersin Street, Hanoi, Vietnam.
authors.forename: Hang L K
authors.initials: HL
authors.lastname: Nguyen
authors.forename: Reiko
authors.initials: R
authors.lastname: Saito
authors.forename: Ha K
authors.initials: HK
authors.lastname: Ngiem
authors.forename: Makoto
authors.initials: M
authors.lastname: Nishikawa
authors.forename: Yugo
authors.initials: Y
authors.lastname: Shobugawa
authors.forename: Doan C
authors.initials: DC
authors.lastname: Nguyen
authors.forename: Long T
authors.initials: LT
authors.lastname: Hoang
authors.forename: Lien P
authors.initials: LP
authors.lastname: Huynh
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Suzuki
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000145:classification / Q000302:isolation & purification*; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification / Q000302:isolation & purification*; D009981:Influenza B virus / Q000145:classification / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology; D008297:Male; D009305:Nasopharynx / Q000821:virology; D011159:Population Surveillance; D014744:Vietnam / Q000453:epidemiology