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 thrombo embol ic events in vaccinated individuals. 3 According to the European Medicines Agency (EMA), 30 cases of thrombo embol ic events (predominantly venous) had been reported by March 10, 2021, among the approxi mately 5 million recipients of the Oxford–AstraZeneca COVID-19 vaccine in the European Economic Area. 3 The EMA subsequently stated that “The number of thromboembol ic 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 thrombo embolism. 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. Using data from the Danish National Patient Registry, we then identified all first-time cases of venous thromboembolism in the general adult population in this period (corresponding to the available data period). We focused on venous thrombo embol ism because the thrombo embol ic events reported in relation to the Oxford–AstraZeneca COVID-19 vaccine by March 10, 2021, were predominantly venous, according to publicly available data on EudraVigilance. 3 We followed all individuals from Jan 1, 2010, or their 18th birthday (whichever came first), until their first incident venous thromboembolism (see definition below), death, emigration, or Nov 30, 2018. Individuals with a diagnosis of venous thromboembol ism before Jan 1, 2010, or their 18th birthday were not included in the analyses. Incident venous thromboembolism was defined as the first primary or secondary inpatient hospital diagnosis or outpatient clinic diagnosis of venous thrombo embol ism. Specifically, the following diagnoses were included in the outcome definition: deep vein thrombosis (International Classification of Diseases version 10 [ICD-10]: I80. 1–3), pulmonary embolism (ICD-10: I26), portal vein thrombosis (ICD-10: I81), hepatic vein thrombosis (ICD-10: I82. 0), thrombophlebitis migrans (ICD-10: I82. 1), embolism or thrombosis of vena cava (ICD-10: I82. 2), embolism or thrombosis of renal vein (ICD-10: I82. 3), mesenteric thrombosis (ICD-10: K55. 0H), cerebral infarction due to non-pyogenic cerebral venous thrombosis (ICD-10: I63. 6), and nonpyogenic thrombosis of intracranial venous system (ICD-10: I67. 6). 5, 8 The diagnoses of venous thromboembolism in the Danish National Patient Registry have a documented high positive predictive value. 9 We then calculated incidence rates for venous thromboembolism (any of the diagnoses listed above) for all Danish adults (aged 18 years or older censored at the 100th birthday) as well as for Danes aged 18–64 years. The 18–64-year age group represents the age group in which the Oxford–AstraZeneca COVID-19 vaccine, due to initial perceptions of limited evidence on its efficacy among those aged 65 years and older, has predominantly been used in most European countries—with the exception of the UK, where the vaccine has also been administered among those aged 65 years and older from the outset. 10, 11 We repeated the analysis restricting outcomes to deep vein thrombosis or pulmonary embolism, as they account for more …