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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.

• Aurora, Colorado, United States
• Cincinnati, Ohio, United States
• Highland Hills, Ohio, United States
• São Paulo, Brazil
• Toronto, Ontario, Canada
• Lille, France
• Montpellier, France
• Ahmedabad, India
• Petah Tikva, Israel
• Guadalajara, Mexico
• Rotterdam, Netherlands
• Kielce, Poland
• Warsaw, Poland
• Altındağ, Turkey (Türkiye)
• Istanbul, Turkey (Türkiye)