Study of Oral Upadacitinib and Subcutaneous/Intravenous Tocilizumab to Evaluate Change in Disease Activity, Adverse Events and How Drug Moves Through the Body of Pediatric and Adolescent Participants With Active Systemic Juvenile Idiopathic Arthritis.

Juvenile Idiopathic Arthritis (JIA) is the most common type of arthritis that affects children. The term "idiopathic" means "of unknown origin". It is a chronic (long-lasting) disease that causes swelling, warmth, and pain of one or more small joints. Systemic JIA ia a rare and serious form of JIA. Systemic" means it may affect not only the joints but other parts of the body, including the liver, lungs and heart. sJIA is more severe and can be more challenging to diagnose and treat than other types of juvenile idiopathic arthritis. It is a lifelong disease for many patients and can continue into adulthood. This study will assess how safe and effective upadacitinib is in treating pediatric and adolescent participants aged 1 to < 18 with systemic juvenile idiopathic arthritis (sJIA) and will include a tocilizumab treatment arm for reference. Adverse events and change in the disease activity will be assessed.

Upadacitinib is an investigational drug being developed for the treatment of sJIA. Participants are assigned to 1 of 2 cohorts. In cohort 1, participants will receive upadacitinib or tocilizumab reference. In cohort 2, participants will receive upadacitinib. Approximately 90 participants with sJIA will be enrolled in approximately 45 sites worldwide.

Participants will receive upadacitinib oral tablets once daily or oral solution twice daily or tocilizumab subcutaneous injection or intravenous infusion as per local label for 52 weeks and followed for approximately 30 days.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits/calls during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, checking for side effects and completing questionnaires.