Additional Study Information: The purpose of this global study is to provide evidence of the efficacy and safety of 210 mg dose of tezepelumab administered every 4 weeks subcutaneously in adults and adolescents (12 years of age and older) with a history of asthma exacerbations and severe uncontrolled asthma receiving medium or high dose inhaled corticosteroids plus at least one additional asthma controller medication with or without oral corticosteroids. The primary and key secondary endpoints (lung function and asthma control) are well accepted measures for a study in severe asthma. These endpoints have been shown in the Phase 2b study to clearly differentiate the tezepelumab benefit from placebo. In order to avoid bias the study will be randomized and double blinded. Subject entry will be stratified by region and age (adolescents and adults) to ensure equitable distribution for analysis. Given that TSLP is an upstream and pleiotropic cytokine, the blockade of TSLP with this medicine is anticipated to have broad impact on the spectrum of inflammatory responses seen in asthma. Due to the mechanism of action it is expected that severe asthmatics irrespective of their phenotype of asthma would benefit from treatment with tezepelumab.