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 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 × 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.

Abbreviations: ACIP, Advisory Committee on Immunization Practices, CDC, Centers for Disease Control and Prevention, CDPHE, Colorado Department of Public Health and Environment, CPT, Current Procedural Terminology, FV, Fully vaccinated, HMO, Health maintenance organization, HR, Hazard ratio, ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification, ILI, Influenza-like illness, PV, Partially vaccinated, RSV, Respiratory synctial virus, UV, Unvaccinated