title: Do intensive care data on respiratory infections reflect influenza epidemics?
abstract: OBJECTIVE
  Severe influenza can lead to Intensive Care Unit (ICU) admission. We explored whether ICU data reflect influenza like illness (ILI) activity in the general population, and whether ICU respiratory infections can predict influenza epidemics.
  
  
  METHODS
  We calculated the time lag and correlation between ILI incidence (from ILI sentinel surveillance, based on general practitioners (GP) consultations) and percentages of ICU admissions with a respiratory infection (from the Dutch National Intensive Care Registry) over the years 2003-2011. In addition, ICU data of the first three years was used to build three regression models to predict the start and end of influenza epidemics in the years thereafter, one to three weeks ahead. The predicted start and end of influenza epidemics were compared with observed start and end of such epidemics according to the incidence of ILI.
  
  
  RESULTS
  Peaks in respiratory ICU admissions lasted longer than peaks in ILI incidence rates. Increases in ICU admissions occurred on average two days earlier compared to ILI. Predicting influenza epidemics one, two, or three weeks ahead yielded positive predictive values ranging from 0.52 to 0.78, and sensitivities from 0.34 to 0.51.
  
  
  CONCLUSIONS
  ICU data was associated with ILI activity, with increases in ICU data often occurring earlier and for a longer time period. However, in the Netherlands, predicting influenza epidemics in the general population using ICU data was imprecise, with low positive predictive values and sensitivities.
authors.affiliation: Department of Medical Informatics, Academic Medical Center, Amsterdam, The Netherlands.
authors.forename: Antonie
authors.initials: A
authors.lastname: Koetsier
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Liselotte
authors.initials: L
authors.lastname: van Asten
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Frederika
authors.initials: F
authors.lastname: Dijkstra
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Wim
authors.initials: W
authors.lastname: van der Hoek
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Bianca E
authors.initials: BE
authors.lastname: Snijders
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Cees C
authors.initials: CC
authors.lastname: van den Wijngaard
authors.affiliation: Department of Statistics, Mathematical Modeling and Data Logistics, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Hendriek C
authors.initials: HC
authors.lastname: Boshuizen
authors.affiliation: NIVEL, Netherlands Institute for Health Services Research, Dutch Sentinel General Practice Network, Utrecht, The Netherlands.
authors.forename: Gé A
authors.initials: GA
authors.lastname: Donker
authors.affiliation: Department of Intensive Care, University Medical Center, Utrecht, The Netherlands.
authors.forename: Dylan W
authors.initials: DW
authors.lastname: de Lange
authors.affiliation: Department of Medical Informatics, Academic Medical Center, Amsterdam, The Netherlands.
authors.forename: Nicolette F
authors.initials: NF
authors.lastname: de Keizer
authors.affiliation: Department of Medical Informatics, Academic Medical Center, Amsterdam, The Netherlands.
authors.forename: Niels
authors.initials: N
authors.lastname: Peek
mesh_terms: D000368:Aged; D004196:Disease Outbreaks; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D009980:Influenza A virus / Q000472:pathogenicity*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D011159:Population Surveillance; D012141:Respiratory Tract Infections / Q000453:epidemiology* / Q000821:virology