Influenza vaccine effectiveness and confounding factors among young children

Abstract: This study, done during the 2002–2003 season among children <6 years of age to investigate influenza vaccine effectiveness and confounding factors, involved 2913 children (1512 vaccinees, 1401 non-vaccinees) recruited from 54 paediatric clinics. Between December 2002 and April 2003, parents reported their children''s maximum body temperatures weekly. Influenza-like illness (ILI) was defined as an acute febrile illness (≥38.0°C) during the peak epidemic period. Adjusted odds ratios (ORs) for ILI were obtained using a logistic regression model. In analysis for total subjects, the ORs were significantly decreased for vaccinees (OR: 0.76, 95% CI: 0.66–0.88) and significantly increased for younger age groups, including children aged 2.0–3.9 years (1.42, 1.18–1.72) and those <2.0 years (2.02,1.61–2.54), compared to those between 4.0 and 5.9 years. ORs were significantly increased for children who visited a physician within the last 6 months for a cold (1.27, 1.08–1.50), attended preschool (1.72, 1.45–2.04), and had ≥3 siblings (1.42, 1.15–1.74). These confounding factors are suggested to be considered in estimating vaccine effectiveness among young children. In subgroup analysis by age groups, significantly decreased ORs were seen in 2.0–3.9-year-old (0.59, 0.47–0.74) and 4.0–5.9-year-old (0.75, 0.58–0.98) vaccinees; no significant vaccine effectiveness was detected for those <2.0 years (1.07, 0.80–1.44). Thus, among very young children vaccine effectiveness could not be demonstrated. [Copyright &y& Elsevier]