A Phase IIa Study of Intravenous Rituximab in Pediatric Participants With Severe Granulomatosis With Polyangiitis (Wegener's) or Microscopic Polyangiitis

Inclusion Criteria: * Diagnosis of GPA (EULAR/PRINTO/PRES 2008, Ankara criteria for childhood Wegener's granulomatosis) or diagnosis of MPA (according to the Chapel Hill Consensus Conference) * Newly diagnosed participants or participants with relapsing disease according to the following definition: The recurrence or new onset of potentially organ- or life-threatening disease (i.e. one or more major Birmingham Vasculitis Activity Score for Wegener's Granulomatosis \[BVAS/WG\] items or disease severe enough to require treatment with cyclophosphamide) * For participants of reproductive potential (males and females), use of reliable means of contraception throughout the study participation * For all eligible participants mandatory prophylactic treatment for Pneumocystis jirovecii infection Exclusion Criteria: * Diagnosis of Churg-Strauss syndrome, as defined by the Chapel Hill Consensus Conference * Limited disease that would not normally be treated with cyclophosphamide * Severe disease requiring mechanical ventilation due to alveolar hemorrhage * Requirement for plasmapheresis or dialysis at screening * Incomplete recovery from recent surgery or less than (\<) 12 weeks since surgery prior to baseline or planned within 24 weeks of baseline * Lack of peripheral venous access * Pregnancy or breast-feeding * Evidence of other significant uncontrolled concomitant disease, or of disorder or condition that, in the investigator's opinion, would preclude or interfere with particip…