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 with documented history of any clinically significant cardiac damage prior to Screening (Visit 1) that, in the opinion of the investigator, would impact the participant's participation during the study
Participants with a history of or current lymphoma, Participants with current malignancy or previous history of cancer in remission for less than 12 months prior to Screening (Visit 1)
Participants who are not responsive to OCS based on clinical response or blood eosinophil counts.
Participants who have previously received mepolizumab in the 4 months prior to enrolment (Visit 2)
Participants receiving non-oral systemic corticosteroids in the 4-week period prior to enrolment (Visit 2).
Participants who have received any other monoclonal antibodies within 30 days or 5 half-lives, whichever is longer, of enrolment (Visit 2).
Participants who have received treatment with an investigational agent (biologic or non-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 received limited, accelerated, or full authorization/approval, and are only in use as part of a clinical trial.
Participants who are currently participating in any other interventional clinical study
Participants with any history of hypersensitivity to any monoclonal antibody (including mepolizumab).
Evidence of clinically significant abnormality in the hematological, biochemical, or urinalysis screen from the sample collected at Screening (Visit 1), that could put the participant's safety at risk by participating in the study, as judged by the investigator