 Mepolizumab is an anti-interleukin-5 IL-5 monoclonal antibody that neutralizes IL-5 and reduces eosinophil counts in both sputum and blood. Omalizumab an anti-immunoglobulin E IgE monoclonal antibody mAb is effective in the treatment of moderate to severe allergic asthma. The aim of this study is to investigate whether subjects not optimally controlled on their current omalizumab treatment, who are eligible for therapy with mepolizumab can be effectively and safely switched to treatment with mepolizumab to improve asthma control. The study will provide data on the efficacy, safety, immunogenicity, and tolerability of mepolizumab when switched directly from omalizumab without any wash-out. The learnings from this study may help guide physicians when substituting one biologic with another for the treatment of patients with severe eosinophilic asthma. 