title: Association between the Severity of Influenza A(H7N9) Virus Infections and Length of the Incubation Period.
abstract: BACKGROUND
  In early 2013, a novel avian-origin influenza A(H7N9) virus emerged in China, and has caused sporadic human infections. The incubation period is the delay from infection until onset of symptoms, and varies from person to person. Few previous studies have examined whether the duration of the incubation period correlates with subsequent disease severity.
  
  
  RESULTS
  We analyzed data of period of exposure on 395 human cases of laboratory-confirmed influenza A(H7N9) virus infection in China in a Bayesian framework using a Weibull distribution. We found a longer incubation period for the 173 fatal cases with a mean of 3.7 days (95% credibility interval, CrI: 3.4-4.1), compared to a mean of 3.3 days (95% CrI: 2.9-3.6) for the 222 non-fatal cases, and the difference in means was marginally significant at 0.47 days (95% CrI: -0.04, 0.99). There was a statistically significant correlation between a longer incubation period and an increased risk of death after adjustment for age, sex, geographical location and underlying medical conditions (adjusted odds ratio 1.70 per day increase in incubation period; 95% credibility interval 1.47-1.97).
  
  
  CONCLUSIONS
  We found a significant association between a longer incubation period and a greater risk of death among human H7N9 cases. The underlying biological mechanisms leading to this association deserve further exploration.
authors.affiliation: Department of Biology, Ecole Normale Supérieure de Lyon, 15 parvis René Descartes, 69007 Lyon, France.|WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Victor
authors.initials: V
authors.lastname: Virlogeux
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Juan
authors.initials: J
authors.lastname: Yang
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Vicky J
authors.initials: VJ
authors.lastname: Fang
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Luzhao
authors.initials: L
authors.lastname: Feng
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Tim K
authors.initials: TK
authors.lastname: Tsang
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Jiang
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Peng
authors.initials: P
authors.lastname: Wu
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Jiandong
authors.initials: J
authors.lastname: Zheng
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Eric H Y
authors.initials: EH
authors.lastname: Lau
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Ying
authors.initials: Y
authors.lastname: Qin
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Zhibin
authors.initials: Z
authors.lastname: Peng
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: J S Malik
authors.initials: JS
authors.lastname: Peiris
authors.affiliation: Division of Infectious Disease, Key Laboratory of Surveillance and Early-Warning on Infectious Disease, Chinese Center for Disease Control and Prevention, 155# Changbai Road, Beijing, 102206, China.
authors.forename: Hongjie
authors.initials: H
authors.lastname: Yu
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Cowling
mesh_terms: D003643:Death; D005260:Female; D006801:Humans; D064766:Influenza A Virus, H7N9 Subtype / Q000502:physiology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D013997:Time Factors