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.
Participants must have a diagnosis of systemic Juvenile Idiopathic Arthritis (sJIA) as defined by International League of Associations for Rheumatology (ILAR) criteria with onset before the age of 16 years
Participants must have at least 2 active joints at screening and baseline
Cohort 1 (IL-6 inhibitor therapy naive): Participants who are at least 1 year and less than 18 years of age, except in countries that restrict use of tocilizumab in participants less than 2 years of age
Cohort 2 (open-label baricitinib): Participants who are at least 1 year and less than 18 years of age
Participants must not have polyarticular JIA (positive or negative for rheumatoid factor), extended oligoarticular JIA, enthesitis-related JIA, or juvenile psoriatic arthritis
Participants must not have persistent oligoarticular arthritis as defined by the ILAR criteria
Participants must not have a history or presence of any autoimmune inflammatory condition other than JIA
Participants must not have active anterior uveitis or are receiving concurrent treatment for anterior uveitis