Inactivated influenza vaccine does not reduce all cause respiratory illness in children with pre-existing medical conditions

BACKGROUND: The effectiveness of inactivated influenza vaccine (IIV) immunization in preventing all cause respiratory illness (RI) in children with pre-existing medical conditions has not been fully established and varies from season to season. This study aims to quantify the overall impact of IIV immunization on primary care attended RI episodes in children with pre-existing medical conditions, using robust observational data spanning twelve influenza seasons.

METHODS: Electronic records of IIV eligible children aged 6 months to 18 years were extracted from primary care databases over the years 2004-2015. IIV eligibility criteria according to Dutch guidelines included (chronic) respiratory and cardiovascular disease and diabetes mellitus. For each year, information on IIV immunization status, primary care attended RI episodes (including influenza, acute respiratory tract infections and asthma exacerbations) and potential confounders were collected. Generalized estimating equations were used to model the association between IIV status and occurrence of at least one RI episode during the influenza epidemic period with "current year immunized" as reference group. Robustness of findings were assessed by performing various sensitivity analyzes in which (i) seasons with a mismatch between the dominant circulating influenza virus and vaccine strain were excluded, (ii) influenza periods were further restricted to weeks with at least 30% influenza virus positive specimens in sentinel surveillance (instead of 5%), (iii) propensity scores were used to adjust for confounding.

RESULTS: In total, 11,797 children (follow-up duration: 38,701 child-years) were eligible for IIV for ≥ one season with 29% immunized at least once. The adjusted odds for primary care attended RI episodes during the influenza epidemic period did not differ between current season immunized versus not immunized children (adjusted OR:1.01; 95%CI:0.90-1.13). The various sensitivity analysis showed comparable results.

CONCLUSIONS: IIV immunization in children with pre-existing medical conditions does not reduce all cause RI episodes encountered in primary care during the influenza season.