A Research Study on How Well Concizumab Works for You if You Have Haemophilia A or B With or Without Inhibitors

This study will test how well a new medicine called concizumab works for participants who have haemophilia A or B with or without inhibitors. The purpose is to show that concizumab can prevent bleeds and is safe to use. Participants will have to inject the study medicine every day under the skin with a pen-injector. The study will last for at least 2 years and up to about 4 years. The length of time the participant will be in the study depends on if the study medicine will be available for purchase in their country.

Inclusion Criteria: * Informed consent/assent obtained before any study-related activities. Study-related activities are any procedures that are carried out as part of the study, including activities to determine suitability for the study. * Diagnosis of congenital severe haemophilia A (FVIII below 1%) or moderate/severe congenital haemophilia B (FIX (coagulation factor IX) below or equal to 2%), or congenital haemophilia with inhibitors. * For arm 1 only: Male aged below 12 years of age at the time of signing informed consent. * For arm 1 only: Patients with inhibitors (haemophilia A with inhibitors or haemophilia B with inhibitors) 1. Patients with HAwI (haemophilia A with inhibitors) with historical medical records of a total of at least 26 weeks of on-demand treatment (On-demand or PPX treatment qualifying for this study is understood as patient-treatment solely for bleeds with intravenous coagulation factor-containing products) within the last 52 weeks prior to enrolment (For patients below 1 year of age that have been diagnosed with haemophilia \