The purpose of this multicenter, randomized, placebo-controlled and double-blind study is to evaluate the efficacy and safety of subcutaneous anifrolumab compared with placebo on the overall disease activity in participants with moderate to severe Idiopathic Inflammatory Myopathies (IIM) [polymyositis (PM) or dermatomyositis (DM)] while receiving standard of care (SoC) treatment.

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

Qualified researchers can request access to anonymized individual patient-level data from AstraZeneca group of companies sponsored clinical trials via the request portal. All request will be evaluated as per the AZ disclosure commitment: https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

Yes, indicates that AZ are accepting requests for IPD, but this does not mean all requests will be shared.

AstraZeneca will meet or exceed data availability as per the commitments made to the EFPIA Pharma Data Sharing Principles. For details of our timelines, please rerefer to our disclosure commitment at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

When a request has been approved AstraZeneca will provide access to the de-identified individual patient-level data in an approved sponsored tool . Signed Data Sharing Agreement (non-negotiable contract for data accessors) must be in place before accessing requested information. Additionally, all users will need to accept the terms and conditions of the SAS MSE to gain access. For additional details, please review the Disclosure Statements at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.