Mepolizumab for Chronic Rhinosinusitis with Nasal Polyps

Inclusion Criteria: * Bilateral NP, as diagnosed by endoscopy or historical CT scan * At least one NP surgery\* within the last 10 years. * Severe NP symptoms consistent with a need for surgery (obstruction VAS symptom score\>5, overall, VAS symptom score \>7, endoscopic bilateral NP score ≥4 \[with a score ≥2 in each nasal cavity\]). * Ongoing treatment with INCS (via spray or intranasal liquid steroid wash/douching) for ≥4 weeks prior to screening * ≥2 of the following CRS symptoms for at least 12 weeks: * Nasal blockage/obstruction/congestion * Nasal discharge (anterior/posterior nasal drip) * Facial pain/pressure * Reduction or loss of sense of smell Exclusion Criteria: * If as a result of a medical interview, physical examination, or screening investigation the physician responsible considers the participant unfit for the study. * Cystic fibrosis * Eosinophilic granulomatosis with polyangiitis (also known as Churg Strauss syndrome), Young's, Kartagener's or dyskinetic ciliary syndromes * Antrochoanal polyps * Nasal septal deviation occluding one nostril * Acute sinusitis or upper respiratory tract infection (URTI) at screening or 2 weeks prior to screening * Ongoing rhinitis medicamentosa (rebound or chemical-induced rhinitis) * Participants who have undergone any intranasal and/or sinus surgery (for example polypectomy, balloon dilatation or nasal stent insertion) within 6 months prior to V1 * Participants where NP surgery is contraindicated in the opinion of the Investigator * Participants with a known medical history of HIV infection. * Participants with a known, pre-existing parasitic infestation within 6 months prior to Visit 1. * Participants who are currently receiving or have received within 3 months (or 5 half-lives - whatever is the longest) prior to the screening visit, radiotherapy, or investigational medications/therapies. * Participants with a history of sensitivity to any of the study medications, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK Medical Monitor, contraindicates their participation. Aspirin-sensitive participants are acceptable. * Participants with a history of allergic reaction to anti-IL-5 or other monoclonal antibody therapy. * Use of systemic corticosteroids (including oral corticosteroids) within 4 weeks prior to screening or planned use of such medications during the double-blind period * Treatments with biological or immunosuppressive treatment (other than Xolair) treatment within 5 terminal phase half-lives of Visit 1 * Omalizumab (Xolair) treatment in the 130 days prior to Visit 1 * Commencement or change of dose of allergen immunotherapy within the previous 3 months. * Pregnancy: Women who are pregnant or lactating or are planning on becoming pregnant during the study. Contraceptive use will be required with a double barrier method or documented effective surgical sterilization. * Immunocompromised subjects from disease or medication, other than oral corticosteroids. * Women of childbearing potential (WOCBP) will be included with measures to prevent accidental exposure to IMP by using double barrier contraception and pregnancy test prior to injection.