Study to Characterise Pharmacokinetics (PK), Pharmacodynamics (PD), and Safety of Anifrolumab Administered Subcutaneously (SC) or Intravenously (IV)

Inclusion Criteria: * Participants who have a diagnosis of adult SLE according to the European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) 2019 criteria for ≥ 24 weeks prior to signing the informed consent form (ICF). * "Clinical" Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) ≥ 4 points at Screening. * Antinuclear antibody (ANA)-positive at Screening. * Must be on stable background standard therapy with oral corticosteroids (OCS), antimalarials, and/or immunosuppressants alone or in combination. Exclusion Criteria: * Active severe or unstable neuropsychiatric SLE. * Active severe SLE-driven renal disease. * Known history of primary immunodeficiency, splenectomy, or any underlying condition predisposing to infection, or a positive result for Human Immunodeficiency Virus (HIV) infection confirmed by central laboratory at Screening. * At Screening, confirmed positive test for hepatitis B serology or positive test for hepatitis C antibody. * Any severe case of herpes zoster infection at any time prior to Week 0 (Day 1). * Opportunistic infection requiring hospitalisation or IV antimicrobial treatment within 3 years of randomisation * History of cancer, apart from: 1. Squamous or basal cell carcinoma of the skin treated with documented curative success ≥ 3 months prior to Day 1. 2. Cervical cancer in situ treated with apparent curative success ≥ 1 year prior to Day 1.