Open-Label, Randomized Study With a Tocilizumab Reference Arm to Evaluate Safety, Efficacy and Pharmacokinetics of Baricitinib in Children From 1 to Less Than 18 Years of Age With Systemic Juvenile Idiopathic Arthritis.

The reason for this study is to see if the study drug baricitinib is safe and effective in participants from 1 year to less than 18 years old with systemic juvenile idiopathic arthritis (sJIA). Participants are assigned to 1 of 2 cohorts. In cohort 1, participants will receive baricitinib or tocilizumab reference. In cohort 2, participants will receive baricitinib.