- Asthma Exacerbation: Any asthma exacerbation requiring a change in maintenance asthma therapy in the 6 weeks prior to Visit 1. Note: Subjects who experience an asthma exacerbation in the 6 weeks prior to Visit 1 are not explicitly excluded at Visit 1 provided that, at the Investigator's discretion, the subject's condition is stable and they are considered appropriate for enrolment into this study of up to 12 months' duration.