Socio-demographic indicators gathered in the intake survey and comparison with national statistics.

1students, home-makers, unemployed, long-term sick-leave or parental leave, retired, others. 2% of health-care workers in over-18 French population, indicated as a possible match with the % of GrippeNet.fr participants having contacts with patients. GrippeNet.fr indicators are adjusted by age and gender. In some cases we considered only a partial definition of the variable, with respect to the full definition available in the survey (see File S1) in order to match available national statistics.