Inclusion criteria :

• Male and female patients aged ≥1 and ≤17 years (or country specified age requirement, ≥6 to ≤17 years for Russia) at the time of the screening visit.
• Diagnosis of systemic JIA subtype according to the International Associations against Rheumatism (ILAR) 2001 Juvenile Idiopathic Arthritis (JIA) Classification Criteria OR According to 2024 EULAR/PReS recommendation at Screening.
• Patient with an inadequate response to current treatment and considered as a candidate for a biologic disease modifying anti rheumatic drug (DMARD) as per investigator's judgment.

Exclusion criteria:

• Body weight <10 kg or >60 kg for patients enrolled in the ascending dose cohorts, then body weight <10 kg for patients subsequently enrolled at the selected dose.
• Uncontrolled severe systemic symptoms and/or Macrophage Activation Syndrome (MAS) within 6 months prior to screening.
• History of or ongoing interstitial lung disease, pulmonary hypertension, pulmonary alveolar proteinosis.
• If nonsteroidal anti-inflammatory drugs (NSAIDs) (including cyclo oxygenase-2 inhibitors [COX-2]) taken, dose stable for less than 2 weeks prior to the baseline visit and/or dosing prescribed outside of approved label.
• If non-biologic DMARD taken, dose stable for less than 6 weeks prior to the baseline visit or at a dose exceeding the recommended dose as per local labeling.
• If oral glucocorticoid taken, dose exceeding equivalent prednisone dose 1 mg/kg/day (or 60 mg/day) within 3 days prior to baseline.
• Use of parenteral or intra-articular glucocorticoid injection within 4 weeks prior to baseline.
• Prior treatment with anti-interleukin 6 (IL-6) or IL-6 receptor (IL-6R) antagonist therapies, including but not limited to tocilizumab or sarilumab.
• Treatment with any biologic treatment for sJIA within 5 half-lives prior to the first dose of sarilumab (the required off treatment periods and procedures may vary according to local requirements).
• Treatment with a Janus kinase inhibitor within 4 weeks prior to the first dose of sarilumab; and treatment with growth hormone within 4 weeks prior to the first dose of sarilumab (the required off treatment periods and procedures may vary according to local requirements).
• Treatment with any investigational biologic or non-biologic product within 8 weeks or 5 half-lives prior to baseline, whichever is longer.
• Exclusion related to tuberculosis.
• Exclusion criteria related to past or current infection other than tuberculosis.
• Any live, attenuated vaccine within 4 weeks prior to the baseline visit, such as varicella-zoster, oral polio, rubella vaccines. Killed or inactive vaccine may be permitted based on the Investigator's judgment.
• Exclusion related to history of a systemic hypersensitivity reaction to any biologic drug and known hypersensitivity to any constituent of the product.
• Laboratory abnormalities at the screening visit (identified by the central laboratory).
• Severe cardiac disease due to sJIA.
• Pregnant or breast-feeding female adolescent patients.

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.

History of severe allergic reactions to study medication Currently pregnant or breastfeeding Recent participation in another clinical trial within the last 30 days Presence of uncontrolled medical conditions that could affect safety