Implemented in Switzerland in November 2016, Grippenet provides Internet-based participatory surveillance of influenza-like illness (ILI). The aim of this research is to test the feasibility of such a system and its ability to detect risk factors and to assess ILI-related behaviors. Participants filled in a web-based socio-demographic and behavioral questionnaire upon registration, and a weekly symptoms survey during the influenza season. ILI incidence was calculated weekly, and risk factors associated to ILI were analyzed at the end of each season. From November 2016 to May 2019, 1247 participants were included. The crossing of the Sentinel System (Sentinella) epidemic threshold was associated with an increase or decrease of Grippenet ILI incidence, within the same week or earlier. The number of active users varied according to ILI incidence. Factors associated with ILI were: ages 0-4 compared with 5-14 (adjusted odds ratio (AOR) 0.6, 95% confidence interval (CI) 0.19-0.99), 15-29 (AOR 0.29, 95% CI 0.15-0.60), and 65+ (AOR 0.38, 95% CI 0.16-0.93); female sex (male AOR 0.81, 95% CI 0.7-0.95); respiratory allergies (AOR 1.58, 95% CI 1.38-1.96), not being vaccinated (AOR 2.4, 95% CI 1.9-3.04); and self-employment (AOR 1.97, 95% CI 1.33-3.03). Vaccination rates were higher than those of the general population but not high enough to meet the Swiss recommendations. Approximately, 36.2% to 42.5% of users who reported one or more ILIs did not seek medical attention. These results illustrate the potential of Grippenet in complementing Sentinella for ILI monitoring in Switzerland.
Keywords: influenza; influenza-like illness; participatory surveillance; syndromic surveillance.