Abstract
BackgroundBoth 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.MethodsWe 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.ResultsThe 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.ConclusionOur 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.
Highlights
Both temperature and humidity may independently or jointly contribute to the risk of influenza infections
The symptoms preceding the identification of influenza started on average three days before seeking medical attention. 69% (45/65) reported outdoor training, 76% (33/43) physical exercise and 72% (31/43) feeling cold during the previous three days
An improved understanding of influenza virus transmission is of importance in order to enhance the accuracy of surveillance systems, to have more precise predictions on influenza epidemics and pandemics in the future, and eventually to develop better disease-control intervention strategies
Summary
Both temperature and humidity may independently or jointly contribute to the risk of influenza infections. There is substantial evidence on the seasonal variation of respiratory morbidity and mortality, which results in increased use of health services and hospital admissions during the winter months [1]. Influenza epidemics constitute a serious public health problem associated with increased morbidity and mortality, especially in high risk populations. Worldwide, these annual epidemics result in about three to five million cases of severe illness, and about 250 000 to 500 000 deaths [2]. Studies from temperate and tropical climates have demonstrated that low temperature [4] and humidity increase the risk of seasonal influenza onset in the winter [5,6,7,8]. On the other hand, may be connected through changes in the virus stability and transmission [13]
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