A Trial to Investigate Benralizumab in Children With Eosinophilic Diseases

Inclusion Criteria: All Cohorts: * Male or female participants must be aged 6 to \< 18 years of age at the time of signing the assent form and their caregiver signing the informed consent form. * Body weight greater than (\>=) 15 kilograms (kg). EGPA Cohort: * Therapy with corticosteroids: The prescribed dose of oral corticosteroids (OCS) (greater than \[\>\] 0.1 milligrams per kilogram per day (mg/kg/day), max dose of 50 milligrams per day (mg/day) must be stable (that is, no adjustment of the dose) for at least 4 weeks prior to baseline (Visit 2). * Immunosuppressive therapy: If receiving immunosuppressive therapy, the dosage must be stable for at least 4 weeks prior to baseline (Visit 2). HES Cohort: * Documented HES diagnosis, defined as history of persistent eosinophilia \>1500 cells/µL without secondary cause on 2 examinations ≥1 month apart and evidence of eosinophil-mediated organ involvement. * Symptomatic active HES, or history of a prior flare, or considered eligible based on disease severity per investigator judgement. * AEC ≥1000 cells/µL at screening (Visit 1). * Documented negative testing for Fip1-like 1 gene fused with the platelet-derived growth factor receptor alpha gene (FIP1L1-PDGFR) fusion tyrosine kinase gene translocation. Exclusion Criteria: All Cohorts: * Any current malignancy or history of malignancy. * History of anaphylaxis to any biologic therapy or vaccine. * Known, pre-existing, clinically significant endocrine, autoimmune, metabolic, neurological, renal, gastrointestinal, hepatic, haematological, respiratory, or any other system abnormalities. * Previous receipt of benralizumab in an interventional clinical study. EGPA Cohort: * Diagnosed with granulomatosis with polyangiitis (previously known as Wegener'granulomatosis) or microscopic polyangiitis. * EGPA relapse: any deterioration in EGPA and/or organ-threatening EGPA that per Investigator judgement renders participants unstable in their EGPA within 3 months prior to screening (Visit 1) and through first administration of IP at baseline (Visit 2). * Life-threatening EGPA: imminently life-threatening EGPA disease within 3 months prior to screening (Visit 1) and through first administration of IP at baseline (Visit 2), as per Investigator judgement. HES Cohort: * Life-threatening HES or HES complications, as judged by the investigator. * Hypereosinophilia of unknown significance (HE-US). * Diagnosis of systemic mastocytosis.