title: Host, weather and virological factors drive norovirus epidemiology: time-series analysis of laboratory surveillance data in England and Wales.
abstract: Norovirus, the most commonly identified cause of both sporadic cases and outbreaks of infectious diarrhoea in developed countries, exhibits a complex epidemiology and has a strong wintertime seasonality. Viral populations are dynamic and evolve under positive selection pressure.
  
  
  METHODS
  Time series-adapted Poisson regression models were fitted to daily counts of laboratory reports of norovirus in England and Wales from 1993 to 2006.
  
  
  RESULTS
  Inverse linear associations with daily temperature over the previous seven weeks (rate ratio (RR) = 0.85; 95% CI: 0.83 to 0.86 for every 1 degrees C increase) and relative humidity over the previous five weeks (RR = 0.980; 95% CI: 0.973 to 0.987 for every 1% increase) were found, with temperature having a greater overall effect. The emergence of new norovirus variants (RR = 1.16; 95% CI: 1.10 to 1.22) and low population immunity were also associated with heightened norovirus activity. Temperature and humidity, which may be localised, had highly consistent effects in each region of England and Wales.
  
  
  CONCLUSIONS
  These results point to a complex interplay between host, viral and climatic factors driving norovirus epidemic patterns. Increases in norovirus are associated with cold, dry temperature, low population immunity and the emergence of novel genogroup 2 type 4 antigenic variants.
authors.affiliation: Gastrointestinal, Emerging and Zoonotic Infections Department, Centre for Infections, Health Protection Agency, London, United Kingdom. ben.lopman@hpa.org.uk
authors.forename: Ben
authors.initials: B
authors.lastname: Lopman
authors.forename: Ben
authors.initials: B
authors.lastname: Armstrong
authors.forename: Christina
authors.initials: C
authors.lastname: Atchison
authors.forename: Jim J
authors.initials: JJ
authors.lastname: Gray
mesh_terms: D017250:Caliciviridae Infections / Q000453:epidemiology* / Q000821:virology; D015986:Confounding Factors, Epidemiologic; D004739:England / Q000453:epidemiology; D005759:Gastroenteritis / Q000453:epidemiology* / Q000821:virology; D054884:Host-Pathogen Interactions; D006801:Humans; D029322:Norovirus / Q000472:pathogenicity*; D016012:Poisson Distribution; D011159:Population Surveillance; D014852:Wales / Q000453:epidemiology; D014887:Weather
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