Study to Evaluate Avacopan in Combination With a Rituximab or Cyclophosphamide-containing Regimen, in Children From 6 Years to < 18 Years of Age With AAV.

Inclusion Criteria: * Male and female children and adolescents from 6 to \< 18 years of age * Clinical diagnosis of granulomatosis with polyangiitis (GPA) or microscopic polyangiitis (MPA), consistent with Chapel-Hill Consensus Conference definitions (Jennette et al, 2013) * Newly diagnosed or relapsed AAV with positive test for anti-PR3 or anti-MPO antibodies * At least 1 PVAS major item, at least 3 PVAS nonmajor items, or at least the 2 renal items of proteinuria and hematuria. * eGFR \> 15 mL/min/1.73 m2 (using modified Schwartz equation per central lab guidelines) * Participants must have a bodyweight of ≥ 15 kg at day 1. Exclusion Criteria: * Any other known multisystem autoimmune disease including eosinophilic granulomatosis with polyangiitis (Churg-Strauss), systemic lupus erythematosus , IgA vasculitis (Henoch-Schönlein), rheumatoid vasculitis, Sjogren's syndrome, anti-glomerular basement membrane disease, or cryoglobulinemic vasculitis * Alveolar hemorrhage requiring invasive pulmonary ventilation support anticipated to last beyond the screening period of the study * Any medical condition requiring or expected to require continued use of immunosuppressive treatments, including corticosteroids that may cause confoundment with study assessments and study conclusions.