- Clinical Benefit: Subjects must have experienced documented clinical benefit to enroll. Subjects must meet the following criteria demonstrating clinical benefit: Subjects enrolled in MEA115588 who received mepolizumab must meet all of the following criteria: a) Subject must have had a reduction in their exacerbation frequency by >=50% during MEA115588. The baseline for comparison is the total number of exacerbations reported in the 12 months prior to screening for MEA115588. b) The investigator response on the "Clinician-Rated Response to Therapy" questionnaire at Visit 10 was either: mildly improved, moderately improved or significantly improved. Subjects enrolled in MEA115588 who received placebo must meet all of the following criteria: c) Subject must have had a reduction in their exacerbation frequency by >=50% during the first 8 months of MEA115661. The baseline for comparison is the total number of exacerbations reported in the 12 months prior to screening for MEA115588; d) The investigator confirms that the subject demonstrated improvement during MEA115661. Subjects enrolled in MEA115575 who received mepolizumab must meet all of the following criteria: e) Subject must have reduced their oral corticosteroid dose by >=50% during MEA115575. The baseline for comparison is the subject's optimized oral corticosteroid (OCS) dose at randomization in MEA115575; f) The investigator response on the "Clinician-Rated Response to Therapy" questionnaire at Visit 9 was either: mildly improved, moderately improved or significantly improved. Subjects enrolled in MEA115575 who received placebo must meet all of the following criteria: g) Subject must have reduced their oral corticosteroid dose at randomization by >=50% in the first 6 months of MEA115661. The baseline for comparison is the subject's optimized OCS dose at randomization in MEA115575; h) The investigator confirms that the subject demonstrated improvement during MEA115661.