This is a 52-week, randomized, placebo-controlled, double-blind, parallel group, multicenter study of depemokimab in adults with uncontrolled HES receiving standard of care (SoC) therapy.

The study will recruit patients with a confirmed diagnosis of HES and who are on stable HES therapy for at least 4 weeks prior to randomization (Visit 2). Eligible participants must have uncontrolled HES with a history of repeated flare (≥2 flares in the previous 12 months) and blood eosinophil count of ≥1,000 cells/ microliter (μL) during Screening. Historical HES flares are defined as documented HES-related worsening of clinical symptoms or blood eosinophil counts requiring an escalation in therapy.

Participants who meet the inclusion and exclusion criteria will be randomized in a 2:1 ratio to receive either depemokimab or placebo while continuing their SoC HES therapy.

Inclusion Criteria:

• Participants who are greater than or equal (>=) 40 kilogram (kg) at Screening Visit 1.
• Participants who have a documented diagnosis of HES prior to Visit 2.
• A history of 2 or more HES flares within the past 12 months prior to Visit 1.
• A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies: a) woman of non-childbearing potential (WONCBP) Or b) woman of childbearing potential (WOCBP) and using a contraceptive method that is highly effective, with a failure rate of less than (<) 1 percentage (%).
• Capable of giving signed informed consent.

Exclusion Criteria:

• Participants with HES disease manifestations which in the opinion of the investigator may put the participant at unacceptable risk from study participation or confound interpretation of efficacy or safety data.
• Participants with chronic or ongoing active infections requiring systemic treatment or a pre-existing parasitic infestation within 6 months prior to Visit 1.
• Participants with a known immunodeficiency (e.g., Human Immunodeficiency Virus [HIV]), other than that explained by the use of OCS or other therapy taken for HES.
• Participants with a history of or current lymphoma.
• Participants with current malignancy or previous history of cancer in remission for less than 5 years prior to Visit 1. Participants that had localized carcinoma (i.e., basal or squamous cell) of the skin which was resected for cure will not be excluded.
• Participants with a haematologic malignancy with hypereosinophilia in which HES is not the primary diagnosis, e.g., chronic myeloid leukaemia, myelodysplastic syndrome, chronic eosinophilic leukaemia-not otherwise specified.
• Cirrhosis or current unstable liver or biliary disease per investigator assessment.
• Participants who have severe or clinically significant cardiovascular disease uncontrolled with standard treatment.
• Participants with current diagnosis of vasculitis.
• Hypereosinophila with no clinical symptoms and/or proof of organ dysfunction.
• Clinical diagnosis of Eosinophilic granulomatosis with polyangiitis (EGPA).
• Participants with an allergy/ intolerance to a monoclonal antibody or biologic, or any of the excipients of the investigational product.
• Participants who have a previous documented failure with anti-interleukin (IL)-5/5R therapy.
• Participants who have received monoclonal antibodies (mAb) within 30 days or 5 half-lives, whichever is longer, prior to Visit 1.
• Participants who test positive for the FIP1L1-PDGFRα fusion gene.
• QT interval corrected for heart rate according to Fridericia's formula (QTcF) ≥450 milliseconds (msec) or QTcF ≥480 msec for participants with Bundle Branch Block at Screening Visit 1.
• Participants who are not responsive to OCS based on clinical response or blood eosinophil counts in the opinion of the Investigator.
• Participants who are pregnant or breastfeeding.

IPD for this study will be made available via the Clinical Study Data Request site.

IPD will be made available within 6 months of publishing the results of the primary endpoints, a key secondary endpoints and safety data of the study.

Access is provided after a research proposal is submitted and has received approval from the Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension can be granted, when justified, for up to another 12 months.

This will be a double-blind study with respect to allocation of depemokimab or placebo to participants. All site staff, participants, and investigator will be blinded.