Thrombotic events after vaccination for covid-19 in Italy: a report from the Italian society on thrombosis and haemostasis registry

Since vaccine induced immune thrombotic thrombocytopenia (VITT) became evident in early 2021, research has focused on characterizing this new pathologic entity. Moreover, data were urgently needed on the association, if any, between all COVID-19 vaccines and thrombotic events (TEs). The study was aimed to collect relevant information on all cases of TEs occurring after COVID-19 vaccination. A national, prospective registry was set up by the Italian Society on Thrombosis and Haemostasis, enrolling consecutive adult patients diagnosed with any TE occurring within 30 days from any COVID-19 vaccine dose, with or without thrombocytopenia. The primary objective was the characterization of TEs and description of its management. The study was approved by the national ethics committee on COVID-19 research and by the participating centers. Among 308 patients included in the registry from February 2021 up to 29 Aug 2022, 276 (89.6%) were diagnosed with venous (266) and/or arterial (33) non-VITT TEs, after a median of 14 days after vaccination. The median age was 60 years, 151 (54.7%) were males, 48 (17.5%) had previous venous or arterial TEs. Within 30 days after diagnosis, 8 (2.9%) patients experienced thrombosis progression/recurrence, 2 (0.7%) major bleeding, 8 (2.9%) died. No differences were found in terms of thrombosis characteristics and progression between non-VITT patients receiving mRNA and adenoviral vector-based vaccines. The remaining 32 patients (10.4%) included in the registry were diagnosed with VITT, after a median of 10 days after vaccination. All of them received adenoviral vector-based vaccines. As compared to non-VITT thrombosis, VITT involved more often both venous and arterial sites (25% vs. 4%, p 0.0002) and unusual sites (62.5% vs. 18.8%, p < 0.0001) and was associated with worse outcomes (thrombosis progression/recurrence 25% vs. 2.9%, major bleeding 34.4% vs. 0.7%, death 18.8% vs. 2.9%, for all comparisons p < 0.0001). Significant differences were found between non-VITT and VITT cases. Non-VITT thromboses occurring after vaccination seem to resemble common thrombosis phenotypes. © The Author(s) 2025.