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 aphysician (such as a rheumatologist, neurologist, or dermatologist) experienced intreatment 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 1standard-of-care treatment by the investigator

• Must be receiving 1 or more of the following protocol-permitted, systemicstandard-of-care treatments: i) glucocorticoids, ii) 1 or 2 of the followingimmunomodulatory drugs: mycophenolate mofetil, azathioprine, oral methotrexate, oraltacrolimus, or oral cyclosporine A

• Regular or as needed treatment with topical use of glucocorticoids are permitted totreat skin lesions on a stable dose for greater than or equal to (>=) 2 weeks priorto first dose of the study drug

• Contraceptive (birth control) use by men or women should be consistent with localregulations regarding the acceptable methods of contraception for thoseparticipating in clinical studies

• Must be medically stable on the basis of clinical laboratory tests performed atscreening. If the results of the clinical laboratory tests are outside the normalreference ranges, the participant may be included only if the investigator judgesthe abnormalities or deviations from normal to be not clinically significant

• Demonstrable muscle weakness at screening and Week 0 measured by the Manual MuscleTesting (MMT)-8 less than or equal to (<=)135 units

• Demonstrable muscle weakness at screening measured by any 2 or more of thefollowings: (i) PhGA greater than or equal to (>=) 1.5 centimeter (cm), (ii) 1 ormore muscle enzymes (Creatine kinase [CK], and aldolase) >=1.4*upper limit of normal (ULN), (iii) Myositis disease activity assessment tool (MDAAT)-Extramuscular GlobalAssessment >=1.5 cm

Exclusion Criteria:

• Has myositis other than PM/DM, including but not limited to amyopathicdermatomyositis (ADM), clinically amyopathic DM, juvenile DM, inclusion bodymyositis (IBM) immune-mediated necrotizing myopathy diagnosed based on muscle biopsyfindings and positive anti-SRP or anti-HMGCR antibody, drug-induced myositis, PMassociated with human immunodeficiency virus (HIV), and muscular dystrophy,congenital myopathy, metabolic myopathy, and mitochondrial myopathy

• Has other inflammatory diseases that might confound the evaluations of efficacy,including but not limited to rheumatoid arthritis (RA), psoriatic arthritis (PsA),systemic lupus erythematosus (SLE), psoriasis, or Crohn's disease

• Has severe respiratory muscle weakness confirmed by the investigator based on theconsultation with a pulmonologist and the measures of respiratory muscle strengthsuch as maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)and/or maximal voluntary ventilation (MVV) measurements and lung capacity such asforced vital capacity (FVC). The results need to be within population appropriatenormal limits

• Has severe muscle damage (Myositis Damage Index-VAS [Muscle Damage] greater than (>) 7 centimeter [cm]), permanent weakness due to a non-IIM cause, or myositis withcardiac dysfunction

• Has glucocorticoid-induced myopathy which the investigator considers the primarycause of muscle weakness

• Has positive test result of anti-melanoma differentiation-associated protein 5 (MDA5) antibody (anti clinically amyopathic dermatomyositis (C-ADM)-140 antibody).

• Has had a nontuberculous mycobacterial infection or opportunistic infection

• Has a history of, or ongoing, chronic or recurrent infectious disease

• Has past history of severe Interstitial lung disease (ILD) flare, severenon-infectious lung inflammation which required active intervention, or multiplerelapses of these conditions

• Presence or history of malignancy within 5 years before screening