A Study of Ustekinumab in Participants With Active Polymyositis and Dermatomyositis Who Have Not Adequately Responded to One or More Standard-of-care Treatments

Inclusion Criteria: * Has a diagnosis of polymyositis (PM)/ dermatomyositis (DM) made or confirmed by a physician (such as a rheumatologist, neurologist, or dermatologist) experienced in treatment of PM/DM at least 6 weeks prior to first dose of the study drug * Has PM or DM which is considered active despite receiving at least 1 standard-of-care treatment by the investigator * Must be receiving 1 or more of the following protocol-permitted, systemic standard-of-care treatments: i) glucocorticoids, ii) 1 or 2 of the following immunomodulatory drugs: mycophenolate mofetil, azathioprine, oral methotrexate, oral tacrolimus, or oral cyclosporine A * Regular or as needed treatment with topical use of glucocorticoids are permitted to treat skin lesions on a stable dose for greater than or equal to (\>=) 2 weeks prior to first dose of the study drug * Contraceptive (birth control) use by men or women should be consistent with local regulations regarding the acceptable methods of contraception for those participating in clinical studies * Must be medically stable on the basis of clinical laboratory tests performed at screening. If the results of the clinical laboratory tests are outside the normal reference ranges, the participant may be included only if the investigator judges the abnormalities or deviations from normal to be not clinically significant * Demonstrable muscle weakness at screening and Week 0 measured by the Manual Muscle Testing (MMT)-8 less than or equal to (\<=)135…