A Study to Investigate Pharmacokinetics, Pharmacodynamics, and Safety of Subcutaneous Anifrolumab in Pediatric Participants 5 to < 18 Years of Age With Systemic Lupus Erythematosus

The purpose of this study is to characterize the pharmacokinetics (PK), pharmacodynamics (PD), and safety of subcutaneous (SC) anifrolumab in pediatric participants with moderate to severe systemic lupus erythematosus (SLE) while on background standard of care (SoC) therapy.

Inclusion Criteria: * Diagnosis of SLE. * Must be receiving at least one of the following SoC regimens for ≥ 4 weeks: oral glucocorticoids (≤1.0 mg/kg/day or ≤ 40 mg/day prednisone equivalent), antimalarials (hydroxychloroquine, chloroquine, or quinacrine), or a single permitted immunosuppressant (azathioprine, mycophenolate mofetil/mycophenolic acid, methotrexate, mizoribine, or tacrolimus) within specified dose limits. * Participant must have moderate to severe active SLE disease defined as Systemic lupus erythematosus disease activity index 2000 (SLEDAI-2K) ≥ 6 total points. * Body weight ≥ 15 kg. * Participants must agree to follow study specific contraception requirements as per local regulations. Exclusion Criteria: * Known diagnosis of an IFN mediated autoinflammatory interferonopathy. * History of, or current diagnosis of, clinically significant non-SLE related vasculitides. * Active, severe SLE-driven renal disease with significant proteinuria. * Active severe or unstable neuropsychiatric SLE including but not limited to aseptic meningitis; cerebral vasculitis; myelopathy; demyelination syndromes (ascending, transverse, acute inflammatory demyelinating polyradiculopathy); acute confusional state; impaired level of consciousness; psychosis; acute stroke or stroke syndrome; cranial neuropathy; status epilepticus; cerebellar ataxia; and mononeuritis multiplex. * In participants ≥ 11 years of age, a history or evidence of suicidal ideation (severity of 4 \[active: method and intent, but no plan\] or 5 \[active: method, intent, and plan\]) within the past 6 months; or any suicidal behavior within the past 12 months or recurrent suicidal behavior in the lifetime of the participant based on an assessment with the Columbia suicide severity rating scale (C-SSRS). * History of, or current diagnosis of, catastrophic antiphospholipid syndrome (APS). * History of recurrent or opportunistic infection requiring hospitalization and intravenous (IV) antibiotics. * Known history of a primary immunodeficiency, splenectomy, or any underlying condition that predisposes the participant to infection, or a positive result for human immunodeficiency virus (HIV) infection. * Active hepatitis B and C infection. * Any active or recent herpes zoster (HZ) infection that has not completely resolved within 12 weeks prior to study entry or that emerges between screening and Day 1. * Any history of severe or recurrent HZ, including non-cutaneous HZ, herpes encephalitis, ophthalmic herpes, or 2 or more prior HZ episodes. * Any cytomegalovirus (CMV) or Epstein Barr virus (EBV) infection that has not completely resolved. * History of cancer. * Prior receipt of anifrolumab. * Prior treatment with directly acting cytotoxic B-cell depleting therapeutics. * A known history of allergy or reaction to any component of the study intervention formulation or history of anaphylaxis to any human gamma globulin therapy, human proteins, or monoclonal antibodies (mAbs).