title: Risk factors associated with severe manifestations of 2009 pandemic influenza A (H1N1) infection in China: a case-control study.
abstract: BACKGROUND
  No studies on the risk factors of 2009 pandemic influenza A (H1N1) in China have been reported. We aimed to investigate the risk factors for severe manifestations of 2009 pandemic H1N1 influenza in China
  
  
  METHODS
  A case-control study with 343 severe hospitalized patients and 343 randomly selected mild controls was conducted. The diagnosis was established by assessment of clinical symptoms and confirmed by the real-time reverse-transcriptase-polymerase chain reaction assay. Severe or mild patients were classified by uniform criteria issued by the Ministry of Health in China.
  
  
  RESULTS
  The multivariable logistic regression analysis showed that the overweight or obese subjects admitted to hospital with H1N1 influenza were more likely to experience severe manifestations. The ORs were 3.70 (95% CI: 2.04-6.72) and 35.61 (95% CI: 7.96-159.21) respectively. Subjects at age less than 5 years or older than 60 years had an increased risk of severe manifestations (OR = 21.14, 95% CI: 7.79-57.33). We also observed increased risk among subjects with longer time interval from symptom onset to hospital admission (OR = 3.26, 95% CI: 2.08-5.11) or peasants (OR = 9.79, 95% CI: 5.11-18.78). Those with chronic disorders had increased risk of severe manifestations of H1N1 influenza.
  
  
  CONCLUSIONS
  We provide evidence on the risk factors associated with severe manifestations of 2009 pandemic H1N1 influenza in a study of hospitalized subjects in China.
authors.affiliation: Shandong Center for Disease Prevention and Control, Jinan, Shandong Province, China.
authors.forename: Yan-yan
authors.initials: YY
authors.lastname: Ren
authors.forename: Yu-yan
authors.initials: YY
authors.lastname: Yin
authors.forename: Wen-qing
authors.initials: WQ
authors.lastname: Li
authors.forename: Yi
authors.initials: Y
authors.lastname: Lin
authors.forename: Ti
authors.initials: T
authors.lastname: Liu
authors.forename: Shuang
authors.initials: S
authors.lastname: Wang
authors.forename: Sheng-yang
authors.initials: SY
authors.lastname: Zhang
authors.forename: Zhong
authors.initials: Z
authors.lastname: Li
authors.forename: Xian-jun
authors.initials: XJ
authors.lastname: Wang
authors.forename: Zhen-qiang
authors.initials: ZQ
authors.lastname: Bi
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D002681:China; D005260:Female; D006760:Hospitalization; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000473:pathology* / Q000821:virology*; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000150:complications; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012307:Risk Factors; D055815:Young Adult