title: Association of early annual peak influenza activity with El Niño southern oscillation in Japan.
abstract: BACKGROUND
  Seasonality characterizing influenza epidemics suggests susceptibility to climate variation. El Niño southern oscillation (ENSO), which involves two extreme events, El Niño and La Niña, is well-known for its large effects on inter-annual climate variability. The influence of ENSO on several diseases has been described.
  
  
  OBJECTIVE
  In this study, we attempt to analyze the possible influence of ENSO on the timing of the annual influenza activity peak using influenza-like illness report data in Japan during 1983-2007.
  
  
  METHODS
  Influenza surveillance data for 25 influenza epidemics, available under the National Epidemiological Surveillance of the Infectious Diseases, was used in this study. ENSO data were obtained from the Japan Meteorological Agency.
  
  
  RESULTS
  Influenza-like illness peak week varied largely during the study period, ranging between 4th and 11th weeks (middle of winter to early spring). The average of peak week during ENSO cycles (n = 11, average = 4.5 +/- 0.9) was significantly earlier than in non-ENSO years (n = 14, average = 7.6 +/- 2.9; P = 0.01), but there was no significant difference in the peak timing between hot (El Niño) and cold (La Niña) phases. Earlier peaks of influenza activity were observed in 16, out of 25, epidemics. These coincided with 10 (90.9%) out of 11 ENSO and 6 (85.7%) out of seven large-scale epidemics.
  
  
  CONCLUSIONS
  Influenza activity peak occurred earlier in years associated with ENSO and/or large scale epidemics.
authors.affiliation: Department of Infectious Disease Control and International Medicine, Division of Public Health, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan. hasanz@med.niigata-u.ac.jp
authors.forename: Hassan
authors.initials: H
authors.lastname: Zaraket
authors.forename: Reiko
authors.initials: R
authors.lastname: Saito
authors.forename: Naohito
authors.initials: N
authors.lastname: Tanabe
authors.forename: Kiyosu
authors.initials: K
authors.lastname: Taniguchi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Suzuki
mesh_terms: D002980:Climate; D004196:Disease Outbreaks; D006801:Humans; D007251:Influenza, Human / Q000453:epidemiology*; D007564:Japan / Q000453:epidemiology; D013997:Time Factors; D014887:Weather