Inclusion Criteria:  Patients with a diagnosis of EGPA independently of ANCA status, Patients aged of 18 years or older, Patients with newly-diagnosed disease or relapsing disease at the time of screening, with an active disease defined as a Birmingham Vasculitis Activity Score (BVAS) ≥3, Patients within the first 21 days following initiation/increase of corticosteroids at a dose ≤ 1 mg/kg/day (pulses of methylprednisolone before oral corticosteroid therapy are authorized) Patients having given their written informed consent prior to participation in the study. Patients affiliated with social security or CMU (profit or being entitled)  Exclusion Criteria:  Patients with GPA, MPA, or other vasculitis, defined by the ACR criteria and/or the Chapel Hill Consensus Conference, Patients with vasculitis in remission of the disease defined as a BVAS <3 Patients with severe cardiac failure defined as class IV in New York Heart Association Patients with acute infections or chronic active infections (including HIV, HBV or HCV and checked in the last 12 months), Patients with active cancer or recent cancer (<5 years), except basocellular carcinoma and prostatic cancer of low activity controlled by hormonal treatment, Pregnant women and lactation. Patients with childbearing potential should have reliable contraception for the 12 months duration of the study, Patients with EGPA who have already been treated with mepolizumab within the previous 12 months, Patients with hypersensitivity to a monoclonal antibody or biologic agent, Patients with contraindication to use mepolizumab, cyclophosphamide, mesna, azathioprine or maintenance therapy used for vasculitis, Patients with other uncontrolled diseases, including drug or alcohol abuse, severe psychiatric diseases, that could interfere with participation in the trial according to the protocol, Patients included in other investigational therapeutic study within the previous 3 months, Patients suspected not to be observant to the proposed treatments, Patients who have white blood cell count ≤4,000/mm3, Patients who have platelet count ≤100,000/mm3, Patients who have ALT or AST level greater that 3 times the upper limit of normal that cannot be attributed to underlying EGPA disease, Patients unable to give written informed consent prior to participation in the study Patients under tutorship or curatorship and protected adults.