This study aims to test if a new medicine called **anifrolumab** is safe and helps people with certain muscle diseases called **Idiopathic Inflammatory Myopathies** (IIM). This includes **polymyositis (PM)** and **dermatomyositis (DM)**, which cause muscle weakness and skin problems. In this study, participants will get either anifrolumab or a **placebo** (a dummy treatment) as a shot, along with their usual treatment. To join, you should be 18 to 75 years old, weigh between 40 and 100 kg, and have a diagnosis of PM or DM. You cannot join if you have other similar diseases or certain infections. The study is double-blind, meaning neither you nor the researchers know if you are getting the real medicine or placebo.

• Study Length: Participants will be required to visit study centers for assessments over a period of time.
• Risks: Like any study, there is a risk of side effects, and participants should discuss these with the study team.
• Compensation: Participants may receive compensation for their time and travel.