A Study to Understand How the Study Medicine Dazukibart Works in People With Idiopathic Inflammatory Myopathies

The purpose of this study is to understand how the study medicine, dazukibart, works in people with active idiopathic inflammatory myopathies (dermatomyositis \[DM\] or polymyositis \[PM\]). Idiopathic inflammatory myopathies are a group of disorders that show inflammation of the muscles used for movement. There are several types of idiopathic inflammatory myopathies, including DM and PM. DM and PM involve weakness of the muscles closest to the center of the body, such as the muscles of the hips, thighs, upper arms, and neck. People with these forms of idiopathic inflammatory myopathies may find it difficult to climb stairs, get up from a seated position, or lift items above their head. People with DM can also have a skin rash. These disorders negatively impact the quality of life and functioning of patients. In addition to the above, these disorders can affect how the lungs and heart work. This study is seeking participants who took part in a DM and PM study with dazukibart before. Some participants will receive study medicine, and some participants will not receive study medicine and only complete safety follow-up. The study medicine will be given as an intravenous (IV) infusion (directly into the veins). This takes about 1 hour, every 4 weeks, from Day 1 to Week 48 (about 12 months) of the study. This will be followed by a safety follow-up period that lasts about 4 months after the last infusion. Participants who receive study medicine will have about 18 study visits at the site over about 16 months. There will also be participants enrolled in this study who will not receive study medicine. Such participants will only take part in safety follow-up visits as they do not want to or are not eligible to receive dazukibart. These participants will not receive study medicine and will have up to 4 study visits at the site every 4 weeks to complete safety follow-up.