A Study to Investigate the Efficacy and Safety of Efgartigimod PH20 SC in Adult Participants With Active Idiopathic Inflammatory Myopathy.

Inclusion Criteria: * Ability to consent in the jurisdiction in which the study is taking place and capable of giving signed informed consent. * A definite or probable clinical diagnosis of idiopathic inflammatory myopathy (IIM) * One of the following medical histories: Diagnosis of dermatomyositis (DM) or juvenile dermatomyositis (JDM), Diagnosis of polymyositis (PM) (including antisynthetase syndrome (ASyS)), Diagnosis of immune-mediated necrotizing myopathy (IMNM) * Diagnosed with active disease as defined by the presence of at least 1 of the following criteria: Abnormal levels of at least 1 of the following enzymes: creatine kinase (CK), aldolase, lactate dehydrogenase, aspartate aminotransaminase (AST), alanine aminotransferase (ALT), based on central laboratory results; Electromyography demonstrating active disease within the past 3 months; Active dermatomyositis (DM) skin rash; Muscle biopsy indicative of active idiopathic inflammatory myopathy (IIM) in the past 3 months; Magnetic resonance imaging within the past 3 months indicative of active inflammation * Muscle weakness * Receiving a permitted background treatment for idiopathic inflammatory myopathy. * Contraceptive use consistent with local regulations, where available, for individuals participating in clinical studies. Women of childbearing potential must have a negative serum pregnancy test during screening and a negative urine pregnancy test at baseline before receiving investigational medicinal product (IMP)…