Subject must have a past history of at least one confirmed EGPA relapse (i.e., requiring increase in oral corticosteroids (OCS) dose, initiation/increased dose of immunosuppressive therapy or hospitalisation) within the past 2 years which occurred at least 12 weeks prior to Screening (Visit 1) whilst receiving a dose of prednisolone (or equivalent) of >=7.5 milligram per day (mg/day). Refractory disease: Either: Failure to attain remission (BVAS=0 and OCS dose <=7.5 mg/day prednisolone or equivalent) within the last 6 months following induction treatment with a standard regimen, administered for at least 3 months. Note: a) Subjects who have received a cyclophosphamide (CYC) induction regimen may be included a minimum of 2 weeks after the last dose of daily oral CYC, or 3 weeks after the last dose of pulsed intravenous CYC prior to Baseline (Visit 2), if their total white blood cells (WBC) is >=4x10^9/Liter (tested at the local laboratory, if necessary) prior to randomisation. b) Subjects who have received a methotrexate, azathioprine, or mycophenolate mofetil induction regimen may be included if on a stable dose for at least 4 weeks prior to Baseline (Visit 2). c) Subjects who have received an induction regimen comprising corticosteroids alone may be included only if they have failed to attain remission after 3 months of treatment AND the corticosteroid dose is >=15 mg/day prednisolone or equivalent for the 4 weeks prior to Baseline (Visit 2). Or: Within 6 months prior to Screening (Visit 1), recurrence of symptoms of EGPA (not necessarily meeting the protocol definition of relapse) whilst tapering OCS, occurring at any dose level >=7.5 mg/day prednisolone or equivalent.