 Mepolizumab is a humanized monoclonal antibody. In conditions where eosinophilia is considered to play an important part in the pathology, including eosinophilic asthma, HES, and eosinophilic granulomatosis with polyangiitis, a consistent reduction in blood eosinophil counts is observed in association with mepolizumab administration, with concomitant clinical improvement. This is a 32-week treatment period, randomized, double-blind, placebo-controlled, parallel group, multicentre study of mepolizumab in adolescent and adult subjects with severe HES receiving standard of care SoC therapy. This study will demonstrate the efficacy of mepolizumab compared with placebo based on maintenance of control of HES symptoms during the treatment period. The study will comprise of a screening period of up to approximately 4 weeks followed by a 32-Week study treatment period subjects will be randomized 1:1 to placebo or mepolizumab and up to 8-week additional follow-up period 12 weeks after the last dose of study treatment. 