Influmeter – an online tool for self-reporting of influenza-like illness in Denmark

Background In October 2013, we implemented 'Influmeter', a web-based influenza-like illness (ILI) self-reporting system, to monitor ILI in the general population in a timely fashion, to provide data for estimations of the burden of influenza and to gain experience with online surveillance systems, in Denmark. After the season 2013/2014 we evaluated the system to decide on its future use. Methods Influmeter study participants provided personal details upon enrolment and reported symptoms weekly within predefined categories. We compared distribution of Influmeter participants with the Danish population, by sex, age, region, chronic diseases and educational level. We calculated the proportion of participants reporting symptoms of ILI weekly and the proportion of Influmeter ILI cases seeking medical assistance, using the Danish and the EU ILI case definitions. Further, we compared timing of increased ILI rates in Influmeter with existing Danish sentinel ILI surveillance using the Danish case definition. Results Compared with the Danish population, Influmeter had more females (p < 0.001) and persons with a higher education (p < 0.001), while the age group 0-24 was under-represented (p < 0.001). Influmeter ILI activity peaked 1 week before the exceeding of the sentinel epidemic threshold. Depending on ILI case definition 16-22% of ILI cases sought medical assistance. Conclusion Influmeter was useful in the timely monitoring of ILI activity in the population that did not seek medical assistance in relation to ILI. We recommend continuation of the system, targeted enrolment of the young and future analyses adjusted for uneven representation relative to the underlying population.