Inclusion Criteria:  Diagnosis of CSU refractory to sgH1-AHs at the time of randomization Demonstrated ability to comply with the required use of the eDiary for the duration of the study For patients on H2 antihistamines or LTRAs for non-CSU indications, treatment at a stable dose for >=2 weeks prior to screening, with no anticipated changes throughout duration of study, including the screening period For women of childbearing potential: agreement to remain abstinent or use contraception For men: agreement to remain abstinent or use a condom, and agreement to refrain from donating sperm  Exclusion Criteria:  Previous participation in a clinical trial of MTPS9579A Chronic urticaria with known cause or other disease with symptoms of urticaria or angioedema Other skin disease associated with chronic itching Uncontrolled disease where flares are commonly treated with systemic corticosteroids History or evidence of any clinically significant medical condition/disease or abnormalities in laboratory tests that, in the investigator's judgment, preclude the patient's safe participation and completion of the study, or interferes with the conduct and interpretation of the study History of anaphylaxis without clearly identifiable avoidable antigen History of anaphylaxis to any biologic therapy for any indication