 The study design incorporates a proportion of participants already established on Mepolizumab and also participants starting on Mepolizumab, allowing recruitment of up to 150 participants in a 6 - 9 month window all sites have large numbers of patients on the waiting list for the drug. Assuming a conservative 0.7 exacerbations per patient per year on mepolizumab ca 160 events in an 18 month period and a minimum rate of attendance of 65 this is 102 observed episodes. The study will be terminated when 100 clinical assessments at exacerbation in total have been completed across all sites for participants on Mepolizumab prior to initiation of rescue treatment. The expected duration of study per participant is 24 months. 