This is an open-label extension study to Study 200622.In this study subjects from Study 200622 will be continued on 4-weekly dosing with open-label mepolizumab 300 milligram (mg) subcutaneously (SC) for an additional 20 Weeks after completing the 32 Week study assessments post-randomization, while they continue with their background HES therapy per standard of care (SoC). Subjects from study 200622 will participate in this extension study if they had completed the 32-Week treatment period in study 200622 or if they were withdrawn from the study pre-maturely, but were continued in the study per protocol until 32 Weeks from randomization. Data from this study (205203) and 200622 will be combined to provide up to 52-Week exposure data to further characterize the long-term safety profile of mepolizumab and provide additional data on the clinical benefit in HES subjects beyond 32 Weeks. The duration of the study participation will be 20 Weeks for subjects who continue with mepolizumab treatment via MHE104317/MHE112562 after this open-label extension study; and 28 Weeks for subjects who do not continue with MHE104317/MHE112562.

Inclusion Criteria:

• Age 12 years and older subjects who were enrolled in Study 200622.
• To be considered for Study 205203, subjects from study 200622 must have completed 32-Week treatment period in the study or if the subject was withdrawn from study treatment prematurely during the 200622 study, but continued in the study per protocol (including HES flare-related assessments) until 32 Weeks from randomization.
• Male or female subjects. Female subjects must be either not a woman of childbearing potential (WOCBP) or WOCBP who agrees to follow the contraceptive guidance at least 30 days prior to the first dose of study treatment and until 16 weeks after the last dose of study treatment.
• The treating physician must confirm a positive benefit/risk ratio. The anticipated clinical benefit from mepolizumab must outweigh any potential safety or tolerability risk in Study 205203.
• Capable of giving signed informed consent.

Exclusion Criteria:

• Subjects with any history of hypersensitivity to any monoclonal antibody (including mepolizumab).
• Subjects with current malignancy or malignancy that developed during Study 200622.
• Subjects who is pregnant or breastfeeding.
• Subjects who has other clinically significant medical conditions uncontrolled with SoC therapy not associated with HES, example (e. g.), unstable liver disease, uncontrolled cardiovascular disease, ongoing active infectious disease.
• Subjects with QT interval corrected (QTc) greater than 450 millisecond (msec) or QTc greater than 480 msec in subjects with bundle branch block based on local Electrocardiogram (EGC) reading.
• Subjects who discontinue study treatment based on liver chemistry stopping criteria during Study 200622.
• Current active liver or biliary disease (with the exception of Gilbert's syndrome or asymptomatic gallstones or otherwise stable chronic liver disease per investigator assessment).
• Subjects 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 the first dose, other than Study 200622 study treatment. The term "investigational" applies to any drug not approved for sale for the disease/indication to treat in the country in which it is being used or investigational formulations of marketed products.
• Subjects who are currently participating in any other interventional clinical study.
• Subjects had an AE (serious or non-serious) considered related to study treatment while participating in Study 200622 which resulted in permanent withdrawal of study treatment.

IPD for this study will be made available via the Clinical Study Data Request site.

IPD will be made available within 6 months of publishing the results of the primary endpoints of the study.

Access is provided after a research proposal is submitted and has received approval from the Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension can be granted, when justified, for up to another 12 months.