Long time trends in influenza-like illness and associated determinants in The Netherlands

We analysed long-term epidemiological trends in influenza-like illness (ILI) in The Netherlands and used an ecological analysis to estimate its relationship with age, influenza vaccination, and virological aspects. This study used data from weekly ILI consultation reports from sentinel general practitioners (1986/1987 to 2006/2007), virological data from sentinel ILI patients (1992/1993 to 2006/2007), and data for influenza vaccine uptake (1991-2005). The incidence of ILI consultations, although varying during the study period, was estimated to decrease in the total population by 12-2/10000 persons each season (95% CI 8-6-15-9). Uptake of influenza vaccination in people aged >= 65 years (elderly) increased from 28% in 1991 to > 70% since 1997. ILI incidence in the elderly declined by 1./10000 persons (P=0-05) per percentage vaccine uptake per season. The decline in ILI incidence over the last 20 years could be related to the increased vaccine uptake. However, insufficient data were present to assess the impact of other potential contributing factors, such as diminished fitness of influenza viruses and changes in consulting behaviour.