A Study of Nipocalimab in Participants With Active Idiopathic Inflammatory Myopathies

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 syn…