This study focuses on patients with Hypereosinophilic Syndrome (HES), a condition where there are persistently high levels of certain white blood cells called eosinophils in the blood, which can cause damage to various organs. The study will test a medication called benralizumab (Fasenra), which is given as an injection under the skin, to see if it can help control HES symptoms and prevent disease flares.

The research aims to determine how effective benralizumab is in delaying or preventing HES flares compared to placebo. During the study, which lasts 24 weeks, participants will receive either benralizumab or placebo through an injection under the skin using a pre-filled syringe. After this initial period, there is an opportunity to continue treatment in an open-label extension phase where all participants receive the active medication.

Throughout the study, doctors will monitor participants’ symptoms, particularly looking for signs of disease worsening or flares. They will also track changes in blood cell counts, fatigue levels, and overall health. The medication being tested is designed to reduce the number of eosinophils in the blood, which may help control the symptoms of HES and prevent organ damage.