A Study to Understand How the Study Medicine (PF-06823859) Works in People With Active Idiopathic Inflammatory Myopathies [Dermatomyositis (DM) and Polymyositis (PM)]

Inclusion Criteria: * Male or female adults (≥18 years old or minimum legal adult age as defined per local regulation, whichever is greater) * Active dermatomyositis (DM) or polymyositis (PM) with age of onset * 18 years old. * Must be receiving a stable dose of standard of care (SOC) background medications at the time of enrollment. Exclusion Criteria: * Myositis due to non-Idiopathic inflammatory myopathies (non-IIM) * Existing diagnosis of inclusion body myositis (IBM) * Presence of immune-mediated necrotizing myositis (IMNM) * Myositis with end-stage organ involvement * Active bacterial, viral or fungal infections or hospitalizations for serious infections within 60 days prior to enrollment * History of recurrent bacterial, viral, fungal, mycobacterial or other infections * Clinically significant finding on a chest x-ray * Have cancer or a history of cancer within 5 years of screening * Significant current or prior disease conditions that may interfere with the response to or safety of the study medicine, including but not limited to: * history of major organ transplant * acute coronary syndrome or any history of significant cerebrovascular disease within 24 weeks of screening * preexisting demyelinating disorder such as multiple sclerosis, or other severe neurological disorder * major surgery within 4 weeks of screening, or scheduled to occur during the study, excluding diagnostic surgery * previous treatment with total lymphoid irradiation * history of any lymphoproliferative disorder such as Epstein Barr Virus, history of lymphoma, leukemia, or symptoms of current lymphatic or lymphoid disease * Clinically significant depression, suicidal ideation, or previous history of suicidal behaviors * Other medical or laboratory abnormality that may increase the risk of study participation * Previous administration with an investigational product (drug or vaccine) within 30 days or of the first dose of study medicine * Current use or incomplete appropriate washout period of any prohibited medication(s) or known exposure to anti-interferon beta (PF-06823859) or any type of anti-interferon beta therapy * Prior SOC medication that does not fulfill the criteria * Certain laboratory results from screening assessments that may interfere with study participation. * Investigator site staff directly involved in the conduct of the study and their family members, site staff and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members