Risk Of Thromboembolism After COVID-19 Vaccination And COVID-19 Infection.

BackgroundVaccine safety monitoring systems worldwide have reported cases of venous thromboembolism and arterial thromboembolism following a COVID-19 vaccination. However, evidence shows that the association between thromboembolism and SARS-CoV-2 infection is stronger, compared to SARS-CoV-2 vaccination. Hence, weighing the risks and benefits of vaccination should also encounter the roles of vaccination in reducing infection rate, and potentially indirectly lowering the risk of thromboembolism caused by infection. MethodsWe conducted a self-controlled case series study (SCCS) from Dec 1st 2020 to 31st August 2022 (before the bivalent vaccine was available) to examinate the association between the first two doses Pfizer/Moderna vaccination and thrombotic events among patients in Corewell Health East (CHE, formerly known as Beaumont Health) healthcare system. We also investigated the effect SARS-CoV-2 infection on the risk of thrombosis events and observed a significant increased risk using the SCCS design. However, because of misclassification bias, SCCS indeed overestimated incidence rate ratio (IRR) of acute event after infection, we then proposed a case-control study addressing this misclassification issues and obtained odd ratio comparing effect of exposure on thrombosis and a subset of controls group. Finally, we analyzed the risk of thromboembolism between vaccinated and unvaccinated groups by a simple diagram, explaining possible factors that affects the probability of experiencing an acute thromboembolism event after a COVID-19 vaccination. ResultsUsing EHR data at Corewell East, we found an increased risk of thrombosis after the first two doses of COVID-19 vaccination, with incidence rate ratios after the first dose is 1.16 (CI: [1.04, 1.29]), and after the second dose of 1.19 (CI: [1.07,1.32]). The association between thromboembolism and SARS-Cov-2 infection depends on prior vaccination status, as the conditional OR among unvaccinated and vaccinated groups are 1.77 (CI: [1.48, 2.1]) and 1.34 (CI: [1.09, 1.66]) respectively. Encountering the vaccine efficacy (VE), receiving the COVID-19 vaccine decreases the risk of thromboembolism, and the benefits of COVID-19 vaccines are much stronger in the period of high infection rate.