Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003-2004 season

Objective: To assess the clinical effectiveness of influenza vaccine in preventing influenza-like illness (ILI) office visits. Study design: 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 peal: 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 versos 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.