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

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 investigational medicinal product 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 investigational medicinal product or 1 of its excipients Development of any malignancy, either new or recurrent, other than basal cell carcinoma of the skin, regardless of relatedness Permanent discontinuation of IMP in ARGX-113-2007, or met the permanent discontinuation criteria at the rollover visit Diagnosis with a deselected subtype of myositis based on the analysis of the phase 2 stage data in ARGX-113-2007, unless the investigator determines that the participant is benefiting from IMP as defined by a score of "much better" or "moderately better" on the 'Clinical Global Impression of Change' and 'Patient Global Impression of Change' assessments for at least 12 weeks, and that enrolling in the study is in the participant's best interest