Comparison of Venous Thromboembolism Outcomes after COVID-19 and Influenza Vaccinations

Background: Published data on the risk of venous thromboembolism (VTE) with COVID-19 vaccines is scarce and inconclusive, leading to an unmet need for further studies. Methods: Retrospective, multicentered study of adult patients vaccinated for one of the three approved COVID-19 vaccines in the United States of America and a pre-COVID-19 cohort of patients vaccinated for influenza at two institutions: Mayo Clinic Enterprise sites and the Medical College of Wisconsin, looking at rate of VTE over 90 days. VTE was identified by applying validated natural language processing algorithms to relevant imaging studies. Kaplan-Meier Curves were used to evaluate rate of VTE and Cox proportional hazard models for incident VTE after vaccinations. Sensitivity analyses were performed for age, sex, outpatient vs inpatient status and type of COVID-19 vaccine. Results: 911,381 study subjects received COVID-19 vaccine [mean age 56.8 (SD 18.3) years, 55.3% females] and 442,612 received influenza vaccine [mean age 56.5 (SD 18.3) years, 58.7% females]. VTE occurred within 90 days in 1,498 (0.11%) of the total 1,353,993 vaccinations: 882 (0.10%) in the COVID-19 and 616 (0.14%) in the influenza vaccination cohort. After adjusting for confounding variables, there was no difference in VTE event rate after COVID-19 vaccination compared to influenza vaccination [adjusted hazard ratio 0.95 (95% confidence interval 0.85-1.05)]. No significant difference in VTE rates was observed between the two cohorts on sensitivity analyses. Conclusion: In this large cohort of COVID-19 vaccinated patients, risk of VTE at 90-days was low and no different than a pre-COVID-19 cohort of influenza vaccinated patients.