title: Populations at risk for severe or complicated influenza illness: systematic review and meta-analysis.
abstract: OBJECTIVE
  To evaluate risk factors for severe outcomes in patients with seasonal and pandemic influenza.
  
  
  METHODS
  Systematic review.
  
  
  METHODS
  Observational studies reporting on risk factor-outcome combinations of interest in participants with influenza. Outcomes included death, ventilator support, admission to hospital, admission to an intensive care unit, pneumonia, and composite outcomes.
  
  
  METHODS
  Medline, Embase, CINAHL, Global Health, and the Cochrane Central Register of Controlled Trials to March 2011.
  Newcastle-Ottawa scale to assess the risk of bias. GRADE framework to evaluate the quality of evidence.
  
  
  RESULTS
  63 537 articles were identified of which 234 with a total of 610 782 participants met the inclusion criteria. The evidence supporting risk factors for severe outcomes of influenza ranged from being limited to absent. This was particularly relevant for the relative lack of data for non-2009 H1N1 pandemics and for seasonal influenza studies. Limitations in the published literature included lack of power and lack of adjustment for confounders was widespread: adjusted risk estimates were provided for only 5% of risk factor-outcome comparisons in 39 of 260 (15%) studies. The level of evidence was low for "any risk factor" (odds ratio for mortality 2.77, 95% confidence interval 1.90 to 4.05 for pandemic influenza and 2.04, 1.74 to 2.39 for seasonal influenza), obesity (2.74, 1.56 to 4.80 and 30.1, 1.74 to 2.39), cardiovascular diseases (2.92, 1.76 to 4.86 and 1.97, 1.06 to 3.67), and neuromuscular disease (2.68, 1.91 to 3.75 and 3.21, 1.84 to 5.58). The level of evidence was very low for all other risk factors. Some well accepted risk factors such as pregnancy and belonging to an ethnic minority group could not be identified as risk factors. In contrast, women who were less than four weeks post partum had a significantly increased risk of death from pandemic influenza (4.43, 1.24 to 15.81).
  
  
  CONCLUSIONS
  The level of evidence to support risk factors for influenza related complications is low and some well accepted risk factors, including pregnancy and ethnicity, could not be confirmed as risks. Rigorous and adequately powered studies are needed.
authors: Department of Medicine, McMaster University, Hamilton, ON, Canada.
authors: Dominik
authors: D
authors: Mertz
authors: Tae Hyong
authors: TH
authors: Kim
authors: Jennie
authors: J
authors: Johnstone
authors: Po-Po
authors: PP
authors: Lam
authors: Michelle
authors: M
authors: Science
authors: Stefan P
authors: SP
authors: Kuster
authors: Shaza A
authors: SA
authors: Fadel
authors: Dat
authors: D
authors: Tran
authors: Eduardo
authors: E
authors: Fernandez
authors: Neera
authors: N
authors: Bhatnagar
authors: Mark
authors: M
authors: Loeb
mesh_terms: D017677:Age Distribution; D002170:Canada / Q000453:epidemiology; D003422:Critical Care / Q000706:statistics & numerical data; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology* / Q000472:pathogenicity; D007252:Influenza Vaccines / Q000008:administration & dosage*; D007251:Influenza, Human / Q000453:epidemiology* / Q000208:ethnology / Q000276:immunology / Q000517:prevention & control*; D008297:Male; D058873:Pandemics; D018579:Patient Selection; D011247:Pregnancy; D011251:Pregnancy Complications, Infectious / Q000517:prevention & control / Q000821:virology*; D012307:Risk Factors; D012621:Seasons; D016896:Treatment Outcome
chemical_list: D007252:Influenza Vaccines
references: Control Clin Trials. 1986 Sep;7(3):177-88
references: 3802833
references: PLoS One. 2010 Oct 05;5(10):
references: 20957195
references: Pediatr Infect Dis J. 2010 Jul;29(7):585-90
references: 20589966
references: BMJ. 2004 Jun 19;328(7454):1490
references: 15205295
references: PLoS One. 2013;8(4):e60348
references: 23565231
references: CMAJ. 2010 Dec 14;182(18):1981-7
references: 21059773
references: JAMA. 1998 Nov 18;280(19):1690-1
references: 9832001
references: JAMA. 2005 Apr 20;293(15):1861-7
references: 15840860
references: PLoS Med. 2013;10(2):e1001380
references: 23393429
references: Clin Infect Dis. 2011 Feb 1;52(3):301-12
references: 21208911
references: Arch Pediatr Adolesc Med. 2010 Nov;164(11):1015-22
references: 21041594
references: PLoS One. 2011;6(10):e25491
references: 22028777
references: Contemp Clin Trials. 2008 Sep;29(5):762-6
references: 18639651
references: Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz. 2010 Dec;53(12):1223-30
references: 21161471
references: Stat Med. 2006 Oct 30;25(20):3443-57
references: 16345038
references: BMC Infect Dis. 2010 Aug 27;10:256
references: 20799934
references: MMWR Recomm Rep. 2010 Aug 6;59(RR-8):1-62
references: 20689501
references: Wkly Epidemiol Rec. 2009 Jul 24;84(30):301-4
references: 19630186
references: N Engl J Med. 2010 Jan 7;362(1):27-35
references: 20032319
references: Pediatr Emerg Care. 2009 Jun;25(6):369-75
references: 19458562
references: J Clin Epidemiol. 2011 Dec;64(12):1283-93
references: 21839614
references: J Clin Epidemiol. 2010 Aug;63(8):926-31
references: 20303710
references: BMJ. 1997 Sep 13;315(7109):629-34
references: 9310563
references: JBI Libr Syst Rev. 2012;10(56 Suppl):1-11
references: 27820287
references: Ann Intern Med. 2012 Apr 3;156(7):512-24
references: 22371849
references: BMJ. 2011 Feb 10;342:d549
references: 21310794
references: CMAJ. 2010 Mar 9;182(4):349-55
references: 20159893
references: Lancet Infect Dis. 2012 Sep;12(9):687-95
references: 22738893
references: Am J Epidemiol. 1998 Dec 1;148(11):1094-102
references: 9850132
references: JAMA. 2010 Apr 21;303(15):1517-25
references: 20407061
references: PLoS Med. 2011 Jul;8(7):e1001053
references: 21750667
references: BMJ. 2003 Sep 6;327(7414):557-60
references: 12958120