Inclusion Criteria:  Age ≥ 18 and ≤ 80 years at Visit 1 Able and willing to provide written informed consent and to comply with the study protocol Severe asthma confirmed after assessment by an asthma physician, requiring treatment with high dose inhaled corticosteroids (ICS) as per BTS criteria [≥1000 micrograms fluticasone propionate equivalent] and ≥ 1 additional drug for asthma [e.g. long acting beta2 agonist (LABA), leukotriene receptor antagonist (LTRA) such as montelukast, theophylline, long acting muscarinic antagonist (LAMA) such as tiotropium) at screening [participants may be included with a lower dose of current ICS at the discretion of the investigator if previous high ICS dose had led to side effects] Adherent with background asthma medication in the opinion of the local MDT Assessed and treatment optimized for any significant asthma-related co-morbidities in the opinion of the local MDT Considered suitable by an asthma specialist for treatment with a monoclonal antibody to block the Interleukin-5 pathway as per local practice, in accordance with NICE TA 431 a recorded blood eosinophil count ≥0.3 x 109/L within the past year history of either ≥ 4 asthma exacerbations requiring high dose oral corticosteroids (exacerbations of asthma in the past year will be defined as worsening of asthma symptoms leading to treatment with prednisolone ≥30 mg oral corticosteroids for ≥3 days as defined by the ERS/ATS Task Force) OR has had continuous oral corticosteroids of at least the equivalent of prednisolone 5 mg per day over the previous 6 months  Exclusion Criteria:  Acute exacerbation requiring high dose oral corticosteroids in the 4 weeks prior to Visit 1. Such patients would be re-assessed when 4 weeks clear of the oral corticosteroid course for re-screening. Long term systemic antibiotic or antifungal therapy Other clinically significant medical disease (including malignancy or immunodeficiency requiring treatment) or uncontrolled concomitant disease that is likely, in the opinion of the investigator, to require a change in therapy or impact the ability to participate in the study Active lung disease other than asthma [Note: Controlled obstructive sleep apnoea (OSA), minor bronchiectasis, asbestos pleural plaques or old (inactive) TB scars are not exclusion criteria]. Patients where an asthma-COPD overlap is suspected by the investigator are not eligible for inclusion History of current alcohol, drug, or chemical abuse or past abuse that would impair or risk the subject's full participation in the study, in the opinion of the investigator Current smoker or smoked in the past 12 month prior to Visit 1 or any history of e-cigarette use Female patients who are pregnant or lactating or planning a family Treatment with any of the following prior to Visit 1 or during the study Any biologic medicine for asthma or an immunomodulating biologic agent for other conditions in the 6 months prior to Visit 1 An investigational agent within 30 days of Visit 1 (or five half-lives of the investigational agent, whichever is longer). Administration of live attenuated vaccine 30 days prior to Visit 1. Other types of vaccines are allowed. Other ongoing immunosuppressive/ immunomodulating therapy [e.g. methotrexate, ciclosporine, azathioprine] other than oral corticosteroids for asthma. Bronchial thermoplasty conducted within 6 months of Visit 1. Patients with helminth infections must be excluded until the infection has been treated Known hypersensitivity to Mepolizumab