AbstractBackgroundInfluenza‐like illness (ILI) incidence estimates in individuals treated with immunosuppressants and/or biologics and/or corticosteroid for an autoimmune or chronic inflammatory disease are scarce. We compared the ILI incidence among immunocompromised population and the general population.MethodWe conducted a prospective cohort study during the 2017–2018 seasonal influenza epidemic, on theGrippeNet.frelectronic platform, which allows the collection of epidemiological crowdsourced data on ILI, directly from the French general population. The immunocompromised population were adults treated with systemic corticosteroids, immunosuppressants, and/or biologics for an autoimmune or chronic inflammatory disease, recruited directly onGrippeNet.frand also among patients of the departments of a single university hospital that were asked to incorporateGrippeNet.fr. The general population consisted of adults reporting none of the above treatments or diseases participating inGrippeNet.fr. The incidence of ILI was estimated on a weekly basis and compared between the immunocompromised population and the general population, during the seasonal influenza epidemic.ResultsAmong the 318 immunocompromised patients assessed for eligibility, 177 were included. During the 2017–2018 seasonal influenza epidemic period, immunocompromised population had 1.59 (95% CI: 1.13–2.20) higher odds to experience an ILI episode, compared to the general population (N= 5358). An influenza vaccination was reported by 58% of the immunocompromised population, compared to 41% of the general population (p< 0.001).ConclusionDuring a seasonal influenza epidemic period, the incidence of influenza‐like illness was higher in patients treated with immunosuppressants, biologics, and/or corticosteroids for an autoimmune or chronic inflammatory disease, compared to the general population.