Inclusion Criteria:  Aged at least 5 years Have given, or their parent/ legal guardian aged ≥ 16 years old has given, written informed consent Diagnosis of NAAV (Appendix 4)  Refractory disease defined by:  Active disease, BVASv3-BIOVAS/ PVAS with ≥ 1 severe (new/worse) or ≥ 3 non-severe (new/worse) items despite 12 weeks of conventional therapy prior to screening visit OR Inability to reduce prednisolone below 15mg/day or (0.2mg/kg/day in case of children) without relapse in the 12 weeks prior to screening visit  Exclusion Criteria:  Previous treatment failure/contraindication to ≥ 2 active trial IMPs Increase in the dose or frequency of background immunosuppressive (e.g. methotrexate) or anti-cytokine therapy within 30 days of screening visit Use of intravenous immunoglobulins within 30 days, or cyclophosphamide or lymphocyte depleting biologic (e.g. rituximab) within 6 months of screening visit Concomitant use of any biologic and/or anti-TNF agent other than the trial IMPs during the trial period Have an active systemic bacterial, viral or fungal infection, or tuberculosis Hepatitis B (HB) core antibody (Ab) or HB surface antigen positive or hepatitis C antibody positive or human immunodeficiency virus (HIV) antibody test positive History of malignancy within five years prior to screening visit or any evidence of persistent malignancy, except fully excised basal cell or squamous cell carcinomas of the skin, or cervical carcinoma in situ which has been treated or excised in a curative procedure Pregnant or breastfeeding, or inability/unwillingness to use a highly effective method of contraceptive if a woman of childbearing potential (WOCBP;see section 11.9) Severe disease, which in the opinion of the physician prevents randomisation to placebo Recent or upcoming major surgery within 45 days of screening visit Leukocyte count < 3.5 x 109 cells/l, platelet count < 100 x 109 cells/l, neutrophil count of < 2 x 109 cells/l ALT or ALP > 3 times the upper limit of normal Symptomatic congestive heart failure (NYHA class III/IV) requiring prescription medication within 90 days of screening visit Demyelinating disorders History or presence of any medical condition or disease which, in the opinion of the Investigator, may place the participant at unacceptable risk because of trial participation Administration of live or live attenuated vaccines within 45 days of screening Have received an investigational medicinal product (IMP) within 5 half-lives or 30 days prior to screening Diagnosis of adenosine deaminase type 2 (DADA2) Hypersensitivity to the active IMP substance or to any of the formulation excipients