This is a multicentre, randomised, double-blind (DB), parallel-group, placebo-controlled, 24-week Phase III study to compare the efficacy and safety of benralizumab versus placebo administered by SC injection Q4W in patients with hypereosinophilic syndrome (HES). This study comprises 2 distinct periods (together defined as the 'main study'): A 24-week, DB treatment period, during which patients will be randomised to receive either benralizumab or placebo, in addition to their prior stable HES background therapy, and an open-label extension (OLE) period, during which all patients will receive benralizumab. The primary database lock (DBL) will occur when approximately 38 patients have had their first HES worsening/flare event during the DB treatment period and all randomised patients have had the opportunity to be followed up for the 24-week DB treatment period. A patient must complete the 24-week DB treatment period on investigational product (IP) to be eligible to enter the OLE treatment period. The final DBL will occur after the last patient completes the OLE.