Approximately 834 patients will be randomised. Patients will be stratified by country/region, age group (adult or adolescent), and peripheral blood eosinophil count at time of Visit 1 (<300 or ≥300 cells/μL).All the patients will be randomised to either placebo or benralizumab (1:1 ratio) for a 48-weeks treatment, every 4 weeks for the first 3 doses and then every 8 weeks thereafter.