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-tropical countries is important for taking actions for pandemic strategies, and links to the WHO global influenza network.
Nguyen
Hang L K
HL
National Institute of Hygiene and Epidemiology, 1 Yersin Street, Hanoi, Vietnam.
Saito
Reiko
R
Ngiem
Ha K
HK
Nishikawa
Makoto
M
Shobugawa
Yugo
Y
Nguyen
Doan C
DC
Hoang
Long T
LT
Huynh
Lien P
LP
Suzuki
Hiroshi
H
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