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 to enrolment (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 the first 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 \[FIP1L1- Platelet-derived Growth Factor Receptor (PDGFRα) (F/P)\] 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 due to viruses, bacteria, and fungi within 4 weeks prior to enrolment (Visit 2) * Participants with a pre-existing parasitic infestation within 6 months prior to enrolment (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 w…