An evidence synthesis approach to estimating the incidence of seasonal influenza in the Netherlands

Objectives To estimate, using Bayesian evidence synthesis, the age-group-specific annual incidence of symptomatic infection with seasonal influenza in the Netherlands over the period 2005-2007. Methods The Netherlands population and age group distribution for 2006 defined the base population. The number of influenza-like illness (ILI) cases was estimated from sentinel surveillance data and adjusted for underascertainment using the estimated proportion of ILI cases that do not consult a general practitioner. The estimated number of symptomatic influenza (SI) cases was based on indirect evidence from the surveillance of ILI cases and the proportions of laboratory-confirmed influenza cases in the 2004/5, 2005/6 and 2006/7 respiratory years. In scenario analysis, the number of SI cases prevented by increasing vaccination uptake within the 65+ age group was estimated. Results The overall symptomatic infection attack rate (SIAR) over the period 2005-2007 was estimated at 25% (95% credible interval [CI]: 21-32%); 410200 SI cases (95% CI: 338500-518600) were estimated to occur annually. Age-group-specific SIARs were estimated for <5years at 49% (21-137%), for 5-14years at 30% (20-47%), for 15-44years at 26% (21-32%), for 45-64years at 19% (14-25%) and for 65+ years at 17% (10-30%). Under assumed vaccination uptake increases of 5% and 15%, 1970 and 5310 SI cases would be averted. Conclusions By synthesising the available information on seasonal influenza and ILI from diverse sources, the annual extent of symptomatic infection can be derived. These estimates are useful for assessing the burden of seasonal influenza and for guiding vaccination policy.