title: Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003-2004 season.
abstract: OBJECTIVE
  To assess the clinical effectiveness of influenza vaccine in preventing influenza-like illness (ILI) office visits.
  
  
  METHODS
  We analyzed billing and immunization registry data for healthy 6- to 21-month-olds from 5 Denver, Colorado pediatric practices (n = 5193). ILI and pneumonia/influenza (a subset of ILI) were defined from International Classification of Diseases, Ninth Revision, Clinical Modification codes for office visits occurring during peak influenza season. Partially vaccinated (PV) and fully vaccinated (FV) patients were defined as having 1 shot and 2 shots, respectively, more than 14 days before the first ILI visit. The likelihood of an ILI visit was determined using a Cox proportional hazards model accounting for patient characteristics, practice site, and immunization status.
  
  
  RESULTS
  A total of 28% of the patients had an ILI office visit, and 5% had a pneumonia/influenza visit. Hazard ratios (HRs) for FV versus UV were 0.31 (95% confidence interval [CI] = 0.3 to 0.4) for ILI and 0.13 (95% CI = 0.1 to 0.2) for pneumonia/influenza, corresponding to a vaccine effectiveness (1 - HR x 100) of 69% for ILI and 87% for pneumonia/influenza. The corresponding HRs for PV versus UV were 1.0 (95% CI = 0.9 to 1.2) and 1.1 (95% CI = 0.8 to 1.5).
  
  
  CONCLUSIONS
  Although 2 doses of vaccine were 69% effective against ILI office visits and 87% effective against pneumonia/influenza office visits, 1 dose did not prevent office visits during the 2003-2004 influenza season.
authors.affiliation: Department of Pediatrics, Colorado Health Outcomes Program, University of Colorado at Denver and Health Sciences Center, Denver, CO, USA. mandy.allison@hsc.utah.edu
authors.forename: Mandy A
authors.initials: MA
authors.lastname: Allison
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Daley
authors.forename: Lori A
authors.initials: LA
authors.lastname: Crane
authors.forename: Jennifer
authors.initials: J
authors.lastname: Barrow
authors.forename: Brenda L
authors.initials: BL
authors.lastname: Beaty
authors.forename: Norma
authors.initials: N
authors.lastname: Allred
authors.forename: Stephen
authors.initials: S
authors.lastname: Berman
authors.forename: Allison
authors.initials: A
authors.lastname: Kempe
mesh_terms: D005260:Female; D006801:Humans; D007223:Infant; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000517:prevention & control*; D008297:Male; D009819:Office Visits / Q000706:statistics & numerical data; D012621:Seasons; D014611:Vaccination / Q000706:statistics & numerical data*
chemical_list: D007252:Influenza Vaccines