Study in Pediatrics With HypEREosinophilic Syndrome (SPHERE)

Inclusion Criteria:

• Participant must be aged 6 to 17 years inclusive, at Screening (Visit 1).

• Participants who have been diagnosed with HES for at least 6 months prior toenrolment (Visit 2).

• A history of 2 or more HES flares within the past 12 months prior to Screening (Visit 1).

• Participants must have blood eosinophil count >=1000 cells per microliter (/mcL)present at Screening.

• Participants must be on a stable dose of HES therapy for the 4 weeks prior to thefirst dose of mepolizumab (Visit 2)

• Male and/or female

• Signed written informed consent

Exclusion Criteria:

• Life-threatening HES or life-threatening HES co-morbidities

• Other concurrent medical conditions that may affect the participant's safety

• Eosinophilia of unknown significance

• Fusion tyrosine kinase gene translocation [FIP1L1- Platelet-derived Growth FactorReceptor (PDGFRα) (F/P)] positivity

• Clinical diagnosis of eosinophilic granulomatosis with polyangiitis (EGPA)

• Participants with chronic or ongoing active infections requiring systemic treatment,as well as participants who have experienced clinically significant infections dueto viruses, bacteria, and fungi within 4 weeks prior to enrolment (Visit 2)

• Participants with a pre-existing parasitic infestation within 6 months prior toenrolment (Visit 2)

• Participants with a known immunodeficiency (e.g. Human immunodeficiency virus [HIV]), other than that explained by the use of OCS or other therapy taken for HES

• Participants with documented history of any clinically significant cardiac damageprior to Screening (Visit 1) that, in the opinion of the investigator, would impactthe participant's participation during the study

• Participants with a history of or current lymphoma, Participants with currentmalignancy or previous history of cancer in remission for less than 12 months priorto Screening (Visit 1)

• Participants who are not responsive to OCS based on clinical response or bloodeosinophil counts.

• Participants who have previously received mepolizumab in the 4 months prior toenrolment (Visit 2)

• Participants receiving non-oral systemic corticosteroids in the 4-week period priorto enrolment (Visit 2).

• Participants who have received any other monoclonal antibodies within 30 days or 5half-lives, whichever is longer, of enrolment (Visit 2).

• Participants who have received treatment with an investigational agent (biologic ornon-biologic) within the past 30 days or 5 drug half-lives, whichever is longer,prior to enrolment (Visit 2).

• Use of candidate Coronavirus disease 2019 (COVID-19) vaccines that have not receivedlimited, accelerated, or full authorization/approval, and are only in use as part ofa clinical trial.

• Participants who are currently participating in any other interventional clinicalstudy

• Participants with any history of hypersensitivity to any monoclonal antibody (including mepolizumab).

• Evidence of clinically significant abnormality in the hematological, biochemical, orurinalysis screen from the sample collected at Screening (Visit 1), that could putthe participant's safety at risk by participating in the study, as judged by theinvestigator