A Phase II Trial to Evaluate the Clinical Efficacy, Safety and Tolerability of MAS825 in Pediatric and Adult Participants With Still's Disease

Inclusion Criteria: * Age ≥ 1 with a diagnosis of Still's Disease * Active diseases defined as: * CRP or ferritin levels greater than ULN, and any of: * Fever ≥ 38°C attributed to Still's Disease activity and documented for a number of days prior to Day 1 or * Rash attributed to Still's Disease activity or * Musculoskeletal involvement: arthritis in a number of joints per ACP criteria for active joint or * Serositis or * Macrophage activation syndrome activity as defined by ferritin levels and at least one of: platelet count, a biomarker or fibrinogen levels attributed to Still's Disease activity by the investigator * Need for glucocorticoids (prednisone or equivalent) Exclusion Criteria: * Patients out of weight range * Ongoing or previous treatment with immunomodulatory drugs * A limited number of Still's Disease patients that have previously received MAS825 through a managed access program are permitted on the study * Glucocorticoid dose exceeding a set limit * Any conditions or significant medical problems which places the patient at unacceptable risk for MAS825 therapy * Still's disease patients with evidence of macrophage activation syndrome are permitted in the study * Still's Disease patients with evidence of interstitial lung disease including those requiring supplemental oxygen therapy are permitted in the study * History of ongoing, chronic or possibly recurrent infection (e.g. HIV, TB, HCV, HBV) and/or symptoms and signs of clinically significant active bacterial, fungal or viral infections * Live vaccinations within a set time prior to MAS825 treatment. Live vaccines are prohibited up to several months following the last dose * History of malignancy of any organ system, including post-transplant lymphoproliferative disorder, treated or untreated, within a number of years, regardless of whether there is evidence of local recurrence and metastases * History of hypersensitivity to any of the study drugs or to drugs of similar chemical classes or to any of the excipients * Pregnant or breastfeeding women * Women of child-bearing potential who do not agree to comply with required contraceptive use Other protocol-defined inclusion/exclusion criteria may apply.