In March 2011, the German sentinel surveillance system for influenza (Arbeitsgemeinschaft Influenza (AGI)) was complemented by an Internet-based syndromic monitoring system (GrippeWeb) for acute respiratory infections (ARI) and influenza-like-illness (ILI). To assess representativeness of GrippeWeb participants, key demographic variables and lifetime prevalence of asthma and diabetes were compared with data from the general population of Germany. To ‘validate’ GrippeWeb, we compared weekly ARI and medically attended ARI (MAARI) rates, generated between weeks 35/2011 and 34/2012, with AGI MAARI rates and overlaid GrippeWeb ILI rates with the number of positive influenza samples obtained by the AGI. GrippeWeb had high weekly participation rates (62% of participants reported in ≥90% of possible weeks). Although it varied by age group, participants reported a mean of between 1.3 and 6.0 ARI episodes and between 0.1 and 2.4 ILI episodes during the study year. Estimated GrippeWeb MAARI incidence was very similar to the AGI MAARI incidence and influenza circulation was reflected well in the GrippeWeb ILI rates. GrippeWeb became a reliable monitoring system shortly after implementation, capturing the burden of ARI and ILI at general population level. The high degree of agreement between GrippeWeb’s and AGI’s MAARI data lends support to the validity of both systems
