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Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a newly recognized syndrome mediated by anti-platelet factor 4 antibodies induced by Covid-19 adenovirus-vectored vaccines including ChAdOx1 nCoV-19 and Ad26.COV2.S. This study validated a proposed Brighton Collaboration case definition for VITT. A data collection form was developed and used to capture the variations in VITT criteria and assess their level of diagnostic certainty from adjudicated positive VITT case datasheets in Germany (n = 71), UK (n = 220), Australia (n = 203), and Taiwan (n = 56). We observed high prevalence of each component of the proposed VITT definition in positive cases (84%-100%), except for the occurrence of thrombosis or thromboembolism criterion in only 34% of VITT cases in Taiwan. The sensitivity of this proposed definition was 100% for Germany and UK, 92% for Australia, and 89% for Taiwan cases. These findings support the validity of this case definition for VITT., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: H. Tran reports project grant funding support outside of the submitted work from AstraZeneca and Sanofi. W.-I. Huang is a member of the national enhanced passive Covid-19 vaccine safety surveillance team in Taiwan. J. Buttery has served as an independent data monitoring committee member for CEPI SPEAC, GSK, Serum Institute of India, SK Biosciences and Clover Vaccines. His employer Murdoch Children’s Research Institute is compensated for his time. A. Greinacher reports grants and non-financial support from Aspen, Boehringer Ingelheim, MSD, Bristol Myers Squibb (BMS), Paringenix, Bayer Healthcare, Gore Inc., Rovi, Sagent, Biomarin/Prosensa, personal fees from Aspen, Boehringer Ingelheim, MSD, Macopharma, BMS, Chromatec, Werfen, nonfinancial support from Boehringer Ingelheim, Portola, Ergomed, GTH e.V. outside the submitted work. No other authors report conflicts of interest. Parts of the German results reported have been obtained in a study conducted by Universitätsmedizin Greifswald under service contract No. EMA/2021/17/TDA. The views expressed are the personal views of the authors and may not be understood or quoted as being made on behalf of or reflecting the position of the European Medicines Agency or one of its Committees or Working Parties. The Taiwan VITT case information was based on data provided by the Vaccine Adverse Event Reporting System, operated by Taiwan Centers for Disease Control., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)