This clinical trial is focused on studying a condition called Systemic Juvenile Idiopathic Arthritis (sJIA), which affects children and adolescents. This is a type of arthritis that causes joint inflammation and can also lead to fever and rash. The study is testing a treatment called sarilumab, which is given as a solution for injection under the skin. Sarilumab is also known by its code name SAR153191/REGN88.

The purpose of the study is to understand how sarilumab behaves in the body of children and adolescents aged 1 to 17 years with sJIA. This will help determine the right dose and schedule for treating this condition. Participants in the study will receive repeated doses of sarilumab, and the study will include an initial phase followed by an extension phase. The study will monitor how the body processes the medication and will also look at the safety and tolerability of the treatment over time.

Throughout the study, researchers will assess various health indicators, such as the number of joints affected by arthritis and overall well-being, to see how the treatment impacts the condition. The study will also track any side effects experienced by participants. The goal is to gather comprehensive information to help improve treatment options for children and adolescents with sJIA.