title: Effect modification of environmental factors on influenza-associated mortality: a time-series study in two Chinese cities.
abstract: BACKGROUND
  Environmental factors have been associated with transmission and survival of influenza viruses but no studies have ever explored the role of environmental factors on severity of influenza infection.
  
  
  METHODS
  We applied a Poisson regression model to the mortality data of two Chinese metropolitan cities located within the subtropical zone, to calculate the influenza associated excess mortality risks during the periods with different levels of temperature and humidity.
  
  
  RESULTS
  The results showed that high absolute humidity (measured by vapor pressure) was significantly (p < 0.05) associated with increased risks of all-cause and cardiorespiratory deaths, but not with increased risks of pneumonia and influenza deaths. The association between absolute humidity and mortality risks was found consistent among the two cities. An increasing pattern of influenza associated mortality risks was also found across the strata of low to high relative humidity, but the results were less consistent for temperature.
  
  
  CONCLUSIONS
  These findings highlight the need for people with chronic cardiovascular and respiratory diseases to take extra caution against influenza during hot and humid days in the subtropics and tropics.
authors.affiliation: Department of Community Medicine, School of Public Health, The University of Hong Kong, Hong Kong Special Administrative Region, 5/F William Mong Block, 21 Sassoon Road, Hong Kong, China.
authors.forename: Lin
authors.initials: L
authors.lastname: Yang
authors.forename: Ping Yan
authors.initials: PY
authors.lastname: Chen
authors.forename: Jian Feng
authors.initials: JF
authors.lastname: He
authors.forename: King Pan
authors.initials: KP
authors.lastname: Chan
authors.forename: Chun Quan
authors.initials: CQ
authors.lastname: Ou
authors.forename: Ai Ping
authors.initials: AP
authors.lastname: Deng
authors.forename: J S
authors.initials: JS
authors.lastname: Malik Peiris
authors.forename: Chit Ming
authors.initials: CM
authors.lastname: Wong
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D002681:China; D002947:Cities; D002980:Climate; D005260:Female; D006801:Humans; D006813:Humidity; D007223:Infant; D007231:Infant, Newborn; D007251:Influenza, Human / Q000401:mortality*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D016019:Survival Analysis; D013696:Temperature; D014505:Urban Population; D055815:Young Adult