Background: The development of COVID-19 vaccines and mass vaccination is a landmark achievement of modern medicine. Management of patients with anti-neutrophil cytoplasmic autoantibodies (ANCA) associated vasculitis (AAV) during the pandemic has been challenging. Immunosuppressive (IS) medications to control vasculitis are associated with severe COVID-19 infection and may impair immune response to the vaccine. In this study we aim to identify correlations between humoral response to the vaccine and IS medications used in the management of AAV. Methods: A multicentre cross-sectional study was carried out across 3 centres, John Hopkins Hospital, Vasculitis centre, Baltimore USA, Royal Preston Hospital, UK and The University Hospital, Munich, Germany. One hundred and fifty-nine patients were included. Serum anti-spike IgG enzyme immunoassays were performed between 11th March 2021 and 19th August 2021. The cut off titre for positive humoral response was between >0.8 to >1.24 u/ml depending on the assays. Results: Of the 159 patients included; mean age 65 ± 14 years, 55% developed detectable SARS-CoV-2 anti-spike antibodies. The use of Rituximab (RTX) was shown to be significant for a poor humoral response to COVID-19 vaccines (p= 0.01). For every 1g increase in the cumulative dose of RTX given, antibody seropositivity reduced by 10% (as shown in Table 1). Multivariate analysis demonstrated CD19 reconstitution was significantly associated with antibody production (p <0.001). Those that received RTX over 6 months prior to vaccination had a seven-fold increase in antibody production. Conclusions: Our study has demonstrated a significant correlation with B cell depleting agents (Rituximab) and SARS-CoV-2 anti-spike antibody production following vaccination. We recommend CD 19 counts as a reliable marker for prediction of antibody production. Decisions to delay or change maintenance treatments should be made on balance with the risk of relapse and other risk factors. We recommend considering other immunosuppressive treatments such as Azathioprine as maintenance treatment, especially in those who are vulnerable to hospitalisation or severe COVID-19 infection. Disclosures: DG is a consultant to ChemoCentryx and Aurinia Inc. The other authors have no other disclosures or competing interests.