Vaccines against Covid-19, venous thromboembolism, and thrombocytopenia. A population-based retrospective cohort study

Background Covid-19 vaccines may increase the risk of venous thromboembolism (VTE), thrombocytopenia (TCP), and VTE associated with TCP. We aimed at estimating this risk by age and sex, after the first dose of both adenovirus vector-based and mRNA-based Covid-19 vaccines, and after the second dose of m-RNA vaccines. Methods In this population-based retrospective cohort study, we examined three groups: 1 662 719 people 10 years of age and over vaccinated with the first dose of a Covid-19 vaccine, 622 778 with the second dose, and 190 616 diagnosed of Covid-19 in the same period (between1 January 2021 and 18 April 2021). The rates of various clinical presentations of VTE and TCP were compared with those in the reference population (7 013 040 people served by the health care system in 2919). The two primary outcomes were the observed 21 day rate of a composite variable of cerebral venous sinus thrombosis, mesenteric thrombosis, portal vein thrombosis, or any venous thromboembolism (VTE) associated with thrombocytopenia (TCP), and the rate of any VTE associated with TCP (VTE+TCP). Analyses were standardised by age and sex. Findings The 21 day rate per 100 000 of the primary composite variable was 2.15 in the reference population, 5.65 following the first vaccine dose (standardised difference, 2.53 (95 percent confidence interval, 1.04-4.00), and 7.23 following the second dose (standardised difference, 4.07 (95 percent confidence interval, 1.43-6.70). The event rates of VTE+TCP and of all the secondary variables showed the same patterns. Excess event rates were higher in men than in women, and they were not especially increased in any particular age group. All Covid-19 vaccines were associated with increased rates of the outcome variables. Excess event rates were many-fold higher in the Covid-19 cohort. Interpretation We observed small increases of rates of venous thromboembolism in usual and unusual anatomical sites and of thrombocytopenia in recipients of both adenovirus vector and mRNA vaccines against Covid-19. Excess rates were higher in men than in women and they were not particularly elevated in any specific age group.