BackgroundMepolizumab, a monoclonal antibody targeting interleukin-5, is of proven clinical benefit in severe asthma; prospective, long-term, real-world data in severe asthma are required. Research QuestionThis study aimed to assess the real-world benefit of 2 years’ mepolizumab treatment in severe asthma. Study Design and MethodsREALITI-A was a 2-year, international, prospective study enrolling adults with asthma, newly initiating mepolizumab 100 mg subcutaneously (physician decision). Outcomes in the 1-year pre-mepolizumab versus 2-year follow-up periods included rates of clinically significant asthma exacerbations (CSE; deterioration requiring systemic corticosteroids and/or emergency department [ED] visit/hospitalisation), exacerbations requiring ED visit/hospitalisation, exacerbations requiring hospitalisation, proportion of patients with no exacerbations, median daily maintenance oral corticosteroid (mOCS) dose, proportion of patients discontinuing mOCS completely, asthma control questionnaire (ACQ)-5 score, forced expiratory volume in 1s (FEV1) and adverse events (AEs). ResultsAfter 2 years’ follow-up, 73% of patients (n=599/822) had no record of mepolizumab discontinuation. During the 2-year follow-up versus pre-mepolizumab period (N=822), rates of CSEs, exacerbations requiring ED visit/hospitalisation, or hospitalisation only were reduced by 74%, 79% and 73%, respectively (odds ratio for no CSEs: 10.0; 95% confidence interval: 7.55, 13.25). Median (Q1, Q3) daily mOCS dose decreased from 10.0 (5.0, 14.7) mg at Week 0 (n=297) to 0.0 (0.0, 5.0) mg at Weeks 101–104 (n=168), and the proportion of patients discontinuing mOCS increased progressively to 43% at 1 year and 57% at 2 years. There was a 1.53-point reduction in ACQ-5 scores from baseline at 2 years. At Months 21–24, least square mean FEV1 improved by 142 mL from baseline. Ninety (11%) and 7 (<1%) patients experienced mepolizumab-related AEs and serious AEs during the follow-up period, respectively. InterpretationIn patients with severe asthma, real-world mepolizumab treatment for 2 years was well-tolerated, and was associated with sustained reductions in exacerbations and progressive reductions in mOCS use.