An Efficacy and Safety Study of Intravenous Anifrolumab to Treat Systemic Lupus Erythematosus in Pediatric Participants

This study aims to characterize the pharmacokinetics, pharmacodynamics, efficacy, and safety of anifrolumab solution for infusion compared with placebo solution for infusion in pediatric participants with severe active systemic lupus erythematosus who are on background standard of care therapy. The study duration for a participant will be approximately 116 weeks, which includes: * Screening period of up to 30 days. * Part A consists of a four-week, double-blind, placebo-controlled, randomised, pharmacokinetic period. * Part B is a double-blind, placebo-controlled, randomised, safety/efficacy period lasting 48 weeks (for rollover participants from Part A) or 52 weeks (for de novo participants). * Part C is a 52-week open-label extension period. * Part D is a safety follow-up period. One safety visit at 12 weeks post last dose.