title: Influenza vaccine effectiveness in preventing outpatient, inpatient, and severe cases of laboratory-confirmed influenza.
abstract: BACKGROUND
  In most seasons, the influenza vaccine is effective in preventing influenza, but it is not clear whether it is equally effective in preventing mild and severe cases. We designed a case-control study to compare the effectiveness of the influenza vaccine in preventing outpatient, inpatient, and severe or fatal cases of laboratory-confirmed influenza.
  
  
  METHODS
  Hospitalized patients (n = 691) with laboratory-confirmed influenza in the 2010-2011 season recruited in 29 Spanish hospitals were individually matched by age, admission/visit date, and province with an outpatient with laboratory-confirmed influenza and an outpatient control. Severe cases were considered those patients admitted to intensive care units or who died in the hospital (n = 177). We compared the influenza vaccine status of controls and outpatient cases, inpatient cases, and severe cases using conditional logistic regression adjusted for potential confounding factors. Severe and nonsevere inpatient influenza cases were compared using unconditional logistic regression. Vaccine effectiveness was (1 - odds ratio) × 100.
  
  
  RESULTS
  Vaccine effectiveness was 75% (adjusted odds ratio [AOR], 0.25; 95% confidence interval [CI], .16-.39) in preventing influenza outpatient cases, 60% (AOR, 0.40; 95% CI, .25-.63) in preventing influenza-associated hospitalizations, and 89% (AOR, 0.11; 95% CI, .04-.37) in preventing severe cases. In inpatients, influenza vaccination was associated with a lower risk of severe influenza (AOR, 0.42; 95% CI, .22-.80).
  
  
  CONCLUSIONS
  Influenza vaccination prevented influenza cases and hospitalizations and was associated with a better prognosis in inpatients with influenza. The combined effect of these 2 mechanisms would explain the high effectiveness of the vaccine in preventing severe cases due to influenza.
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain. jcastilc@navarra.es
authors.forename: Jesús
authors.initials: J
authors.lastname: Castilla
authors.forename: Pere
authors.initials: P
authors.lastname: Godoy
authors.forename: Angela
authors.initials: A
authors.lastname: Domínguez
authors.forename: Iván
authors.initials: I
authors.lastname: Martínez-Baz
authors.forename: Jenaro
authors.initials: J
authors.lastname: Astray
authors.forename: Vicente
authors.initials: V
authors.lastname: Martín
authors.forename: Miguel
authors.initials: M
authors.lastname: Delgado-Rodríguez
authors.forename: Maretva
authors.initials: M
authors.lastname: Baricot
authors.forename: Nuria
authors.initials: N
authors.lastname: Soldevila
authors.forename: José María
authors.initials: JM
authors.lastname: Mayoral
authors.forename: José María
authors.initials: JM
authors.lastname: Quintana
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Galán
authors.forename: Ady
authors.initials: A
authors.lastname: Castro
authors.forename: Fernando
authors.initials: F
authors.lastname: González-Candelas
authors.forename: Olatz
authors.initials: O
authors.lastname: Garín
authors.forename: Marc
authors.initials: M
authors.lastname: Saez
authors.forename: Sonia
authors.initials: S
authors.lastname: Tamames
authors.forename: Tomás
authors.initials: T
authors.lastname: Pumarola
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D017714:Community-Acquired Infections / Q000175:diagnosis / Q000453:epidemiology* / Q000473:pathology / Q000517:prevention & control*; D003428:Cross Infection / Q000175:diagnosis / Q000453:epidemiology* / Q000473:pathology / Q000517:prevention & control*; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000276:immunology*; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology* / Q000473:pathology / Q000517:prevention & control*; D007297:Inpatients; D008297:Male; D008875:Middle Aged; D010045:Outpatients; D011247:Pregnancy; D011379:Prognosis; D013030:Spain / Q000453:epidemiology; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines
keywords: influenza vaccine; influenza virus; severe influenza; vaccine effectiveness