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.