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 old of age. * Clinical diagnosis of Granulomatosis with Polyangiitis (GPA) or microscopic polyangiitis (MPA), consistent with Chapel-Hill Consensus Conference definitions (Jennette et al, 2013). * Positive anti-PR3(Anti-Proteinase 3) or anti-MPO(Anti-Myeloperoxidase) antibody documented at Screening or historically. Historical positivity is acceptable (even if Screening is negative) if supported by verifiable lab source documentation obtained during AAV diagnosis or disease course; use the most recent positive result. * At least 1 PVAS major item, at least 3 PVAS nonmajor items, or atleast the 2 renal items of proteinuria and hematuria. * Estimated glomerular filtration rate (eGFR) of ≥ 15 mL/minute/1.73 m\^2 at screening and day 1. * Participants must have a bodyweight of ≥ 15 kg at day 1. Exclusion Criteria: * Any other known multisystem autoimmune disease including and not limited to eosinophilic granulomatosis with polyangiitis (EGPA, previously, Churg-Strauss disease), systemic lupus erythematosus, IgA vasculitis / Henoch-Schönlein, Purpura, rheumatoid vasculitis, Sjögren's syndrome, anti-glomerular basement membrane disease, or cryoglobulinemic vasculitis. * Renal replacement therapy / plasmapheresis: subjects will be excluded who received, require, or initiate CRRT (continuous renal replacement therapy), hemodialysis, any renal dialysis, or plasmapheresis within 14 days prior to Screening or between Screening and Day 1. * History of kidney transplantation or is anticipated to require renal transplantation during the study. * 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, ongoing treatment with immunosuppressive, therapy (including systemic glucocorticoids) for a non-AAV indication that in the judgment of the investigator, could confound study assessments or interpretation of study results. * Female subjects of childbearing potential must have a negative highly sensitive serum pregnancy test at Screening and a negative sensitive urine pregnancy test, on day 1, with results confirmed prior to the first administration of investigational product. * Known hypersensitivity or contraindication to avacopan, its excipients, or to any investigational product or required concomitant medication used in this study. * Subject likely to not be available to complete all protocol-required study visits or procedures, and/or to comply with all required study procedures (eg, Clinical Outcome Assessments) to the best of the subject and investigator's knowledge. * History or evidence of any other clinically significant disorder, condition, or disease (other than those specified above) that, in the investigator's judgment, would pose an unacceptable risk to subject safety, or interfere with study assessments or completion. The investigator may consult the Amgen medical monitor as needed. The rationale for exclusion and any consultation must be documented in the subject's source record.