A Study to Assess the Long-term Safety and Efficacy of a Subcutaneous Formulation of Efgartigimod in Adults With Active Idiopathic Inflammatory Myopathy

Inclusion Criteria: * Has completed trial ARGX-113-2007 * Being capable of providing signed informed consent and complying with protocol requirements * Agrees to use contraceptive measures consistent with local regulations and women of childbearing potential must have a negative urine pregnancy test at baseline before receiving the study drug Exclusion Criteria: * Intention to have major surgery during the study period or any other medical condition that has arisen since enrollment in study ARGX-113-2007, that in the investigator's opinion, would confound the results of the study or put the participant at undue risk * Known hypersensitivity reaction to the study drug or 1 of its excipients * Development of any malignancy, either new or recurrent, other than basal cell carcinoma of the skin, regardless of relatedness