This study's purpose is to measure the treatment response from efgartigimod PH20 SC compared with placebo in participants with Idiopathic Inflammatory Myopathy (IIM). Participants with the IIM subtypes of dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), or certain other subtypes of polymyositis (PM; including antisynthetase syndrome [ASyS]) will be included in the study. Treatment response will be measured by Total improvement score (TIS). Additional information can be found on https://myositis-study.com/.

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).

The full list of inclusion criteria can be found in the protocol.

Exclusion Criteria:

• A clinically significant active infection at screening
• A COVID-19 polymerase chain reaction (PCR)-positive test before enrollment
• Any other known autoimmune disease that, in the investigator's opinion, would interfere with an accurate assessment of clinical symptoms of idiopathic inflammatory myopathy (IIM) or put the patient at undue risk
• A history of malignancy unless considered cured by adequate treatment, with no evidence of recurrence for ≥ 3 years before the first administration of the investigational medicinal product (IMP). Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer ; Carcinoma in situ of the cervix; Carcinoma in situ of the breast; Incidental histological finding of prostate cancer
• Severe muscle damage
• Glucocorticoid-induced myopathy that the investigator considers the primary cause of muscle weakness or permanent weakness linked to a non-idiopathic inflammatory myopathy (IIM) cause
• Juvenile myositis (JDM) diagnosed > 5 years from screening or juvenile myositis with extensive calcinosis or severe calcinosis.
• Uncontrolled interstitial lung disease or any other uncontrolled idiopathic inflammatory myopathy (IIM) manifestation that, in the opinion of the investigator, would be likely to require treatment with prohibited medication during the study
• Other inflammatory and noninflammatory myopathies: inclusion body myositis, overlap myositis), metabolic myopathies, muscle dystrophies or a family history of muscle dystrophy, drug-induced or endocrine induced myositis, and juvenile myositis (other than juvenile dermatomyositis (JDM))
• Clinically significant disease, recent major surgery or intends to have surgery during the study, or has any other condition in the opinion of the investigator that could confound the results of the trial or put the patient at undue risk
• Known hypersensitivity reaction to investigational medicinal product (IMP) or 1 of its excipients
• Received a live or live-attenuated vaccine less than 4 weeks before screening.
• Positive serum test at screening for active viral infection with any of the following conditions: Hepatitis B virus (HBV); Hepatitis C virus (HCV); HIV
• Participant has previously participated in an efgartigimod clinical trial and received at least 1 dose of investigational medicinal product (IMP).
• Participant is concurrently participating in any other clinical study, including a noninterventional study.
• Participant has a current or history (ie, within 12 months of screening) of alcohol, drug, or medication abuse.
• Participant is pregnant or lactating or intends to become pregnant during the study.
• Participant has severe renal impairment .
• Participant is institutionalized by a court or other governmental order or is in a dependent relationship with the sponsor or investigator.

The full list of exclusion criteria can be found in the protocol.