Influenza vaccine effectiveness among children 6 to 59 months of age during 2 influenza seasons: a case-cohort study.

Objective: To measure vaccine effectiveness (VE) in preventing influenza-related health care visits among children aged 6 to 59 months during 2 consecutive influenza seasons.

Design: Case-cohort study estimating effectiveness of inactivated influenza vaccine in preventing inpatient/outpatient visits (emergency department [ED] and outpatient clinic). We compared vaccination status of laboratory-confirmed influenza cases with a cluster sample of children from a random sample of practices in 3 counties (subcohort) during the 2003-2004 and 2004-2005 seasons.

Setting: Counties encompassing Rochester, New York, Nashville, Tennessee, and Cincinnati, Ohio.

Participants: Children aged 6 to 59 months seen in inpatient/ED or outpatient clinic settings for acute respiratory illnesses and community-based subcohort comparison. Main Exposure Influenza vaccination.

Main Outcome Measures: Influenza vaccination status of cases vs subcohort using time-dependent Cox proportional hazards models to estimate VE in preventing inpatient/ED and outpatient visits.

Results: During the 2003-2004 and 2004-2005 seasons, 165 and 80 inpatient/ED and 74 and 95 outpatient influenza cases were enrolled, while more than 4500 inpatient/ED and more than 600 outpatient subcohorts were evaluated, respectively. In bivariate analyses, cases had lower vaccination rates than subcohorts. However, significant influenza VE could not be demonstrated for any season, age, or setting after adjusting for county, sex, insurance, chronic conditions recommended for influenza vaccination, and timing of influenza vaccination (VE estimates ranged from 7%-52% across settings and seasons for fully vaccinated 6- to 59-month-olds).

Conclusion: In 2 seasons with suboptimal antigenic match between vaccines and circulating strains, we could not demonstrate VE in preventing influenza-related inpatient/ED or outpatient visits in children younger than 5 years. Further study is needed during years with good vaccine match.