title: Decline in temperature and humidity increases the occurrence of influenza in cold climate.
abstract: BACKGROUND
  Both temperature and humidity may independently or jointly contribute to the risk of influenza infections. We examined the relations between the level and decrease of temperature, humidity and the risk of influenza A and B virus infections in a subarctic climate.
  
  
  METHODS
  We conducted a case-crossover study among military conscripts (n = 892) seeking medical attention due to respiratory symptoms during their military training period and identified 66 influenza A and B cases by PCR or serology. Meteorological data such as measures of average and decline in ambient temperature and absolute humidity (AH) during the three preceding days of the onset (hazard period) and two reference periods, prior and after the onset were obtained.
  
  
  RESULTS
  The average temperature preceding the influenza onset was -6.8 ± 5.6°C and AH 3.1 ± 1.3 g/m3. A decrease in both temperature and AH during the hazard period increased the occurrence of influenza so that a 1°C decrease in temperature and 0.5 g decrease per m3 in AH increased the estimated risk by 11% [OR 1.11 (1.03 to 1.20)] and 58% [OR 1.58 (1.28 to 1.96)], respectively. The occurrence of influenza infections was positively associated with both the average temperature [OR 1.10 per 1°C (95% confidence interval 1.02 to 1.19)] and AH [OR 1.25 per g/m3 (1.05 to 1.49)] during the hazard period prior to onset.
  
  
  CONCLUSIONS
  Our results demonstrate that a decrease rather than low temperature and humidity per se during the preceding three days increase the risk of influenza episodes in a cold climate.
authors.forename: Kari
authors.initials: K
authors.lastname: Jaakkola
authors.forename: Annika
authors.initials: A
authors.lastname: Saukkoriipi
authors.forename: Jari
authors.initials: J
authors.lastname: Jokelainen
authors.forename: Raija
authors.initials: R
authors.lastname: Juvonen
authors.forename: Jaana
authors.initials: J
authors.lastname: Kauppila
authors.forename: Olli
authors.initials: O
authors.lastname: Vainio
authors.forename: Thedi
authors.initials: T
authors.lastname: Ziegler
authors.forename: Esa
authors.initials: E
authors.lastname: Rönkkö
authors.forename: Jouni Jk
authors.initials: JJ
authors.lastname: Jaakkola
authors.affiliation: Institute of Health Sciences, University of Oulu, P,O, Box 5000, FI-90014 Oulu, Finland. tiina.ikaheimo@oulu.fi.
authors.forename: Tiina M
authors.initials: TM
authors.lastname: Ikäheimo
mesh_terms: D000293:Adolescent; D000328:Adult; D003081:Cold Climate; D005387:Finland / Q000453:epidemiology; D006801:Humans; D006813:Humidity; D009980:Influenza A virus; D007251:Influenza, Human / Q000453:epidemiology*; D029524:Betainfluenzavirus; D008297:Male; D016017:Odds Ratio; D013696:Temperature; D055815:Young Adult