This is a Phase III, multicenter, open-label clinical study designed to evaluate the efficacy, safety, pharmacokinetics, and pharmacodynamics of emicizumab prophylaxis in participants aged 1 month and above, who have been diagnosed with Type 3 von Willebrand disease (VWD). Participants on prior standard of care (SOC) on-demand therapy will be assessed via a randomized comparison (Arm A - emicizumab prophylaxis and Arm B - continuation of SOC on-demand therapy), while participants on prior SOC prophylactic therapy (Arm C - emicizumab prophylaxis) will be assessed via intra-participant analysis with data obtained from the preceding non-interventional study (NIS), WP45335 (NCT06883240).

Inclusion Criteria:

• Confirmed diagnosis of Type 3 von Willebrand disease (VWD), based on medical records
• Preexisting medical record verifying the status of von Willebrand factor (VWF) inhibitor (positive or negative, including titer if available)
• Adequate hematologic, hepatic, and renal function
• For participants of childbearing potential: agreement to remain abstinent or adhere to the contraception requirements

Additional Inclusion Criteria for Arms A and B:

• Age ≥1 month at the time of signing Informed Consent/Assent Form
• Documented previous use of on-demand therapy with intermittent (less than once a week) on-demand SOC therapy for VWD
• Having ≥2 treated bleeds (except menstrual bleeds) with factor concentrate within 24 weeks prior to enrollment

Additional Inclusion Criteria for Arm C:

• Age ≥2 years at the time of signing Informed Consent/Assent Form
• Documented and confirmed previous use of SOC prophylactic therapy for VWD (1-3 times weekly, as per prescribed dose) as described in the eligibility of Study WP45335
• Have completed all study requirements as defined in the WP45335 protocol for at least 24 weeks

Exclusion Criteria:

• Inherited or acquired bleeding disorder other than Congenital Type 3 VWD
• History of gastrointestinal bleeding within 18 months prior to enrollment, or any previous diagnosis of angiodysplasia
• History of intracranial hemorrhage
• Previous or current treatment for thromboembolic disease or signs of thromboembolic disease
• Other conditions (e.g., certain autoimmune diseases) that may increase risk of bleeding or thrombosis
• History of clinically significant hypersensitivity associated with monoclonal antibody therapies or components of the emicizumab injection
• Use of systemic immunomodulators (e.g., interferon) at enrollment or planned use during the study, with the exception of anti-retroviral therapy

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing