A Study of Nipocalimab in Participants With Active Idiopathic Inflammatory Myopathies

The purpose of this study is to evaluate the efficacy and safety of Nipocalimab versus placebo in participants with active idiopathic inflammatory myopathies (IIM).

Inclusion Criteria: * Disease classification criteria: Participant meets the diagnostic criteria of probable or definite idiopathic inflammatory myopathies (IIM) based on 2017 The European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria for adult IIM at least 6 weeks prior to first administration of the study intervention * If a participant is on regular or as needed treatment with low potency topical glucocorticoids (GC) that are allowed in the study or topical tacrolimus (TAC) to treat skin lesions, the dose and frequency should be stable for greater than or equal to (\>=) 4 weeks prior to first administration of the study intervention as well as maintained at the same dose until Week 52 of the study * Antibody positivity criteria: Any 1 of the myositis-specific antibodies (MSAs) positive: dermatomyositis (DM): anti-Mi-2 (Mi-2/nucleosome remodeling and deacetylase \[NuRD\] complex), anti-transcription intermediary factor 1-Gamma (TIF1-Gamma), anti- nuclear matrix protein 2 (NXP-2), anti-small ubiquitin-like modifier-1 activating enzyme; anti- antimelanoma differentiation-associated gene 5 (MDA-5) antibodies. Or immune-mediated necrotizing myopathy (IMNM): anti- signal recognition particle (SRP) and anti- 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies. Or anti-synthetase syndrome (ASyS): anti- histidyl- ribonucleic acid \[tRNA\] synthetase (Jo-1), anti- threonyl-tRNA synthetase (PL7), anti- alanyl-tRNA synthetase (PL12), anti- isoleucyl-tRNA synthetase (OJ), and anti- glycyl-tRNA synthetase (EJ) antibodies. If all MSAs are negative or more than 1 MSA is positive within different subtypes (defined by the central laboratory) at screening, the tests should be repeated during the screening period. If the same results are observed at retesting, the participant should not be enrolled in the study Exclusion Criteria: * Has a juvenile myositis diagnosis and now \>=18 years old * Has cancer-associated myositis defined as cancer diagnosis within 3 years of myositis diagnosis except for cervical carcinoma in situ and non-melanoma skin cancer (squamous cell carcinoma, basal cell carcinoma of the skin) * Has comorbidities (example, asthma, chronic obstructive pulmonary disease \[COPD\]) which have required 3 or more courses of oral GC within 1 year prior to screening * Has a history of primary immunodeficiency or secondary immunodeficiency not related to the treatment of the participants IIM * Has experienced myocardial infarction (MI), unstable ischemic heart disease, or stroke within 12 weeks of screening