Inclusion Criteria:

• Male or female.
• ≥ 18 years of age.
• Diagnosis of GPA or MPA according to the definitions of the Chapel Hill Consensus Conference. 
• Have at least one "major" item, or at least three other items, or at least two renal items on the Birmingham Vasculitis Activity Score (BVAS) Version 3.0. 
• New or relapsed GPA or MPA that require treatment with CYC or RTX plus GCs.

Exclusion Criteria: 

• Any other multisystem autoimmune disease.
• Requires mechanical ventilation because of alveolar hemorrhage at Screening.
• Have required management of infections, as follows:
  • Chronic infection requiring suppressive anti-infective therapy (such as latent tuberculosis, pneumocystis, aspergillosis, cytomegalovirus, herpes simplex virus, herpes zoster and atypical mycobacteria);
  • Use of intravenous antibacterials, antivirals, anti-fungals, or anti-parasitic agents.
• Known or suspected active drug and/or alcohol abuse.
• Human immunodeficiency virus (HIV), hepatitis B, or hepatitis C viral screening test showing evidence of active or chronic viral infection at Screening or a documented history of HIV, hepatitis B, or hepatitis C.
• One of the following abnormal laboratory findings at Screening:
  • White blood cells < 3500/mm3
  • Platelet count < 120,000/mm3
  • Total bilirubin > 3 times the upper limit of normal (ULN)
  • Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 5 x ULN
• Acute or chronic liver disease.
• Known hypersensitivity to inactive ingredients of the GC capsules.
• History of or active malignancy, lymphoproliferative or myeloproliferative disorder. Individuals with squamous cell or basal cell carcinomas of the skin and individuals with cervical carcinoma in situ who have received curative surgical treatment may be eligible for this study.
• History of anti-glomerular basement membrane (GBM) disease.
• Received CYC or RTX within 12 weeks before screening; if on AZA, MMF, MPS, or MTX at the time of Screening, these drugs must be withdrawn prior to receiving CYC or RTX.
• Received more than 3 g cumulative dose of intravenous GCs within 4 weeks before screening.
• Received an oral daily dose of a GC of more than 10 mg prednisone-equivalent for more than 6 weeks continuously prior to screening.
• Received CD20 inhibitor, anti-tumor necrosis factor treatment, abatacept, alemtuzumab, any other experimental or biological therapy, intravenous immunoglobulin or plasma exchange, antithymocyte globulin, or required dialysis within 12 weeks before Screening.
• Received a live vaccination within 4 weeks before Screening or planned between Screening and Week 24.
• Subjects with a history of tuberculosis.
• Pregnant or lactating.
• Clinically significant abnormal electrocardiogram (ECG) during Screening; e.g., QTcF >450 ms.
• Female subjects of childbearing potential unwilling or unable to use a highly effective method of contraception (pearl index <1%) such as complete sexual abstinence, combined oral contraceptive, vaginal hormone ring, transdermal contraceptive patch, contraceptive implant, or depot contraceptive injection in combination with a second method of contraception such as condom, cervical cap, or diaphragm with spermicide during the study and for at least 4 weeks after last administration of IFX-1 (timeframes for SOC have to be considered as described in the respective Prescribing Information). Male subjects with female partners of childbearing potential unwilling to use contraception (condoms) during treatment and for at least 4 months after last administration of treatment.
• Evidence or suspicion that the subject might not comply with the requirements of the study protocol.
• Any other factor which, in the investigator’s opinion, is likely to compromise the subject’s ability to participate in the study.
• The subject is an employee or direct relative of an employee at the study site or sponsor.
• The subject is imprisoned or lawfully kept in an institution.
• The subject has participated in an investigational clinical study during the 12 weeks (or five times the half-life of the previous IMP, whichever is longer) before Screening, or plans to participate in another investigational clinical study during their participation in this study.