Thrombotic outcomes in patients in a large clinical enterprise following COVID-19 vaccination.

Vaccination against COVID-19 reduces infection-related mortality. Unfortunately, reports of vaccine-induced immune thrombotic thrombocytopenia (VITT) in individuals administered adenovirus-vector-based vaccines (ChAdOx1 nCoV-19 and Ad26.COV2.S) have spurred side effect concerns. To address vaccine hesitancy related to this, it is essential to determine the incidence of VITT (defined by a 50% decrease in platelet count and positive anti-PF4 immunoassay within 4-28 days after vaccination) among patients administered two doses of an mRNA-based COVID-19 vaccination. We identified a retrospective cohort of 223,345 patients in the Cleveland Clinic Enterprise administered a COVID-19 vaccine at any location in Northeast Ohio and Florida from 12/4/2020 to 6/6/2021. 97.3% of these patients received an mRNA-based vaccination. Patients with: (1) a serial complete blood count both before and after vaccination and (2) a decrease in platelet count of ≥ 50% were selected for chart review. The primary outcome was the incidence of thrombotic events, including venous thromboembolism (VTE) and arterial thrombosis, 4-28 days post vaccination. Of 74 cohort patients with acute thrombosis, 72 (97.3%) demonstrated clear etiologies, such as active malignancy. Of two patients with unprovoked thrombosis, only one had findings concerning for VITT, with a strongly positive anti-PF4 antibody assay. In this large, multi-state, retrospective cohort, of 223,345 patients (97.2% of whom received the mRNA-based mRNA-1273 or BNT162b2 vaccines), we detected a single case that was concerning for VITT in a patient who received an mRNA vaccine. The overwhelming majority of patients with a thrombotic event 4-28 days following vaccination demonstrated clear etiologies.