Thromboembolism and the Oxford–AstraZeneca COVID-19 vaccine: side-effect or coincidence?

By mid March, 2021, vaccination against COVID-19 using the ChAdOx1 nCoV-19 (AZD1222) vaccine from Oxford–AstraZeneca1,2 was paused in a number of European countries due to reports of thromboembolic events in vaccinated individuals.3 According to the European Medicines Agency (EMA), 30 cases of thromboembolic events (predominantly venous) had been reported by March 10, 2021, among the approximately 5 million recipients of the Oxford–AstraZeneca COVID-19 vaccine in the European Economic Area.3 The EMA subsequently stated that “The number of thromboembolic events in vaccinated people is no higher than the number seen in the general population”.4 To inform the ongoing discussion on the safety of the Oxford–AstraZeneca COVID-19 vaccine, we analysed nationwide population-based data from Denmark to estimate the natural incidence of venous thromboembolism.5 Denmark has a tax-supported universal health-care system,6 in which all hospital contacts are registered in the Danish National Patient Registry.7 We first used the Danish Civil Registration System6 to identify all Danes who were at least 18 years old between Jan 1, 2010, and Nov 30, 2018. [...]detailed clinical descriptions of the thromboembolic events reported in relation to Oxford–AstraZeneca COVID-19 vaccinations are still lacking.4 We are, however, aware that although a substantial fraction of the thromboembolisms seem to be venous, reports are emerging of rare types of multiple thrombosis, bleeding, and thrombocytopenia, apparently similar to disseminated intravascular coagulation, occurring in otherwise healthy individuals shortly after receiving the Oxford–AstraZeneca COVID-19 vaccine.13 These outcomes are not included in the present analysis. [...]our estimated weekly and monthly venous thromboembolism case numbers in the population of 5 million individuals are based entirely on incidence rates from Denmark and might not be representative of the other countries where the Oxford–AstraZeneca COVID-19 vaccine has been used.