A Study to Investigate the Efficacy and Safety of Anifrolumab Administered as Subcutaneous Injection and Added to Standard of Care Compared With Placebo Added to Standard of Care in Adult Participants With Idiopathic Inflammatory Myopathies (Polymyositis and Dermatomyositis)

Capable of giving informed consent.

Inclusion Criteria:

1. 18 - 75 years old
2. Body weight ≥40 kg
3. Must have "probable" or "definite" diagnosis of PM or DM according to the 2017 ACR/EULAR classification criteria for adult myositis.
4. Moderate or severe disease activity per core set measurements.
5. Currently receiving oral prednisone or other polymyositis or dermatomyositis treatments at a stable dose.
6. No history of active tuberculosis or severe COVID-19.
7. Male and female participants must follow contraception guidelines.

Exclusion Criteria:

1. Participants with documented inclusion body myositis (IBM), immune mediation necrotizing myositis (IMNM), juvenile myositis (if diagnosed within 10 years prior to signing the ICF), drug-induced myositis, cancer associated myositis, amyopathic DM, and non inflammatory myopathies (eg, muscular dystrophies).
2. PM and DM patients at a high risk of malignancy.
3. Participants with rapidly progressive interstitial lung disease.
4. Participants with severe muscle damage or permanent weakness due to non-PM or non-DM conditions (i.e. stroke) as per the investigator's opinion.
5. Any history of severe case of herpes zoster infection
6. History of cancer (except adequately treated basal cell carcinoma or cervical cancer in-situ), immunodeficiency, HIV, HBV, active HCV .
7. Any clinical cytomegalovirus or Epstein-Barr virus infection that has not completely resolved within 12 weeks prior to signing the ICF.
8. Opportunistic infection requiring hospitalization or IV antimicrobial treatment within 3 years prior to randomization.
9. Recent non-opportunistic infection requiring hospitalization or anti-infective treatment.
10. Recent or concurrent enrollment in another clinical study with an investigational product.
11. Lactating, breastfeeding, or pregnant females or females who intend to become pregnant or begin breastfeeding