Ten-year performance of Influenzanet: ILI time series, risks, vaccine effects, and care-seeking behaviour

• ILI incidences from Influenzanet and ECDC correlate well over multiple seasons.
• Northern Europeans visit their GP later and less often than Southern Europeans.
• Young age, chronic disease, living with children, and pets are risk factors for ILI.
• Using public transport daily and living with adults not linked with higher ILI risk.
• Influenza vaccines confer moderate and season-specific protection from ILI.

Recent public health threats have propelled major innovations on infectious disease monitoring, culminating in the development of innovative syndromic surveillance methods. Influenzanet is an internet-based system that monitors influenza-like illness (ILI) in cohorts of self-reporting volunteers in European countries since 2003. We investigate and confirm coherence through the first ten years in comparison with ILI data from the European Influenza Surveillance Network and demonstrate country-specific behaviour of participants with ILI regarding medical care seeking. Using regression analysis, we determine that chronic diseases, being a child, living with children, being female, smoking and pets at home, are all independent predictors of ILI risk, whereas practicing sports and walking or bicycling for locomotion are associated with a small risk reduction. No effect for using public transportation or living alone was found. Furthermore, we determine the vaccine effectiveness for ILI for each season.