title: Seasonality of absolute humidity explains seasonality of influenza-like illness in Vietnam.
abstract: BACKGROUND
  Experimental and ecological studies have shown the role of climatic factors in driving the epidemiology of influenza. In particular, low absolute humidity (AH) has been shown to increase influenza virus transmissibility and has been identified to explain the onset of epidemics in temperate regions. Here, we aim to study the potential climatic drivers of influenza-like illness (ILI) epidemiology in Vietnam, a tropical country characterized by a high diversity of climates. We specifically focus on quantifying and explaining the seasonality of ILI.
  
  
  METHODS
  We used 18 years (1993-2010) of monthly ILI notifications aggregated by province (52) and monthly climatic variables (minimum, mean, maximum temperatures, absolute and relative humidities, rainfall and hours of sunshine) from 67 weather stations across Vietnam. Seasonalities were quantified from global wavelet spectra, using the value of the power at the period of 1 year as a measure of the intensity of seasonality. The 7 climatic time series were characterized by 534 summary statistics which were entered into a regression tree to identify factors associated with the seasonality of AH. Results were extrapolated to the global scale using simulated climatic times series from the NCEP/NCAR project.
  
  
  RESULTS
  The intensity of ILI seasonality in Vietnam is best explained by the intensity of AH seasonality. We find that ILI seasonality is weak in provinces experiencing weak seasonal fluctuations in AH (annual power <17.6), whereas ILI seasonality is strongest in provinces with pronounced AH seasonality (power >17.6). In Vietnam, AH and ILI are positively correlated.
  
  
  CONCLUSIONS
  Our results identify a role for AH in driving the epidemiology of ILI in a tropical setting. However, in contrast to temperate regions, high rather than low AH is associated with increased ILI activity. Fluctuation in AH may be the climate factor that underlies and unifies the seasonality of ILI in both temperate and tropical regions. Alternatively, the mechanism of action of AH on disease transmission may be different in cold-dry versus hot-humid settings.
authors.affiliation: National Institute of Hygiene and Epidemiology, Hanoi, Viet Nam; Wellcome Trust Major Overseas Programme, Oxford University Clinical Research Unit, Hanoi, Viet Nam. Electronic address: phamquangthai@gmail.com.
authors.forename: Pham Quang
authors.initials: PQ
authors.lastname: Thai
authors.affiliation: Wellcome Trust Major Overseas Programme, Oxford University Clinical Research Unit, Hanoi, Viet Nam; MIVEGEC, University of Montpellier, CNRS 5290, IRD 224, Montpellier, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Choisy
authors.affiliation: National Institute of Hygiene and Epidemiology, Hanoi, Viet Nam.
authors.forename: Tran Nhu
authors.initials: TN
authors.lastname: Duong
authors.affiliation: National Institute of Hygiene and Epidemiology, Hanoi, Viet Nam.
authors.forename: Vu Dinh
authors.initials: VD
authors.lastname: Thiem
authors.affiliation: National Institute of Hygiene and Epidemiology, Hanoi, Viet Nam.
authors.forename: Nguyen Thu
authors.initials: NT
authors.lastname: Yen
authors.affiliation: National Institute of Hygiene and Epidemiology, Hanoi, Viet Nam.
authors.forename: Nguyen Tran
authors.initials: NT
authors.lastname: Hien
authors.affiliation: Spatial Ecology & Epidemiology Group, Department of Zoology, University of Oxford, Oxford, UK.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Weiss
authors.affiliation: Centre for Tropical Medicine, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK; Wellcome Trust Major Overseas Programme, Oxford University Clinical Research Unit, Ho Chi Minh City, Viet Nam.
authors.forename: Maciej F
authors.initials: MF
authors.lastname: Boni
authors.affiliation: Wellcome Trust Major Overseas Programme, Oxford University Clinical Research Unit, Hanoi, Viet Nam; Centre for Tropical Medicine, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Horby
mesh_terms: D058872:Epidemics; D006801:Humans; D006813:Humidity; D007251:Influenza, Human / Q000453:epidemiology*; D012621:Seasons; D014744:Vietnam / Q000453:epidemiology; D014887:Weather
keywords: Absolute humidity; Influenza-like illness; Regression tree; Seasonality; Vietnam