Maintenance of Remission With Rituximab Versus Azathioprine for Newly-diagnosed or Relapsing Eosinophilic Granulomatosis With Polyangiitis.

Inclusion Criteria: * patients with a diagnosis of EGPA according to Lanham and/or ACR 1990 criteria and/or Revised Chapel Hill Nomenclature and/or MIRRA study inclusion criteria * 18 years of age or more * with newly-diagnosed EGPA or after a vasculitis flare and remission achieved within the past year * independently of ANCA status * within 30-360 days following achievement of vasculitis remission (corresponding to a Birmingham Vasculitis Activity Score (BVAS)=0) achieved with an induction regimen including the one used in the REOVAS trial: either CS alone or in association with CYC (total dose ranging from 4.5-10 g for patients \<65 years old and from 3-10g for patients ≥65 years old) or RTX (2 x 1g (D1, D15) or 4 weekly 375 mg/m2). * with a stable prednisone dose for 30 days or no more prednisone * after oral immunosuppressive drug cessation if started at remission. * Patients included in the REOVAS trial and achieving remission can be included at month 12 visit if they fulfil the other criteria * Patients able to give written informed consent prior to participation in the study. * Affiliation with a mode of social security (profit or being entitled). Exclusion Criteria: * patients with GPA, MPA or other vasculitides * patients with vasculitis not in remission defined as a BVAS \>0 * acute or chronic active infections (including HIV, HBV or HCV) * active or recent cancer ( \<5 years), except basocellular carcinoma and low activity prostatic cancer controlled by hormonal treatment * severe heart failure (New York Heart Association Class IV) or severe, uncontrolled cardiac disease * pregnant women and lactation * patients with childbearing potential will have reliable contraception for all the duration of the study and another 12 months after. Women are considered of childbearing potential (WOCBP), i.e. fertile, following menarche and until becoming post-menopausal unless permanently sterile. Permanent sterilisation methods include hysterectomy, bilateral salpingectomy and bilateral oophorectomy. A postmenopausal state is defined as no menses for 12 months without an alternative medical cause. A high follicle stimulating hormone (FSH) level in the postmenopausal range may be used to confirm a postmenopausal state in women not using hormonal contraception or hormonal replacement therapy. However in the absence of 12 months of amenorrhea, a single FSH measurement is insufficient * men who refuse to use effective method of contraception (condom) from the date of consent through the end of the study * patients who had already been treated with rituximab before the last relapse/flare * patients who have been treated with rituximab with a different induction regimen than 2 x 1g (D1, D14) or 4 weekly 375 mg/m2 infusions * hypersensitivity to a monoclonal antibody or biologics * contraindication to rituximab or azathioprine * other uncontrolled diseases, including drug or alcohol abuse, severe psychiatric diseases, that could interfere with participation * patients included in other investigational therapeutic study within the previous 3 months except in the REOVAS trial, after which patients achieving remission can be included if they fulfil the other criteria * patients suspected not to be observant to the proposed treatments * white blood cell count ≤4,000/mm3 * platelet count ≤100,000/mm3 * ALT or AST level \>3 times the upper limit of normal * patients not able to stop allopurinol and febuxostat which may enhance azathioprine toxicity * patients unable to give written informed consent prior to participation in the study.