Inclusion Criteria:  New diagnosis of AAV as evidenced by relevant biopsy and / or clinical diagnosis in the context of a positive ANCA antibody OR major relapse of previously diagnosed AAV that requires re-induction of remission treatment with intravenous rituximab or cyclophosphamide together with high dose oral corticosteroids CMV seropositive as evidenced by the presence of anti-CMV IgG in the serum Age >18 years Willingness to participate in the study and attend clinic and study visits Able to provide written informed consent  Exclusion Criteria:  Strong suspicion of alternative diagnosis other than AAV Subjects who do not have capacity to consent to study participation as defined by the Mental Capacity Act 2005 Inability or unwillingness to attend study visits