Embolic and thrombotic events after COVID-19 vaccination
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Post-vaccination embolic and thrombotic events, termed vaccine-induced immune thrombotic thrombocytopenia (VITT), vaccine-induced prothrombotic immune thrombocytopenia (VIPIT), thrombosis with thrombocytopenia syndrome (TTS), vaccine-induced immune thrombocytopenia and thrombosis (VITT), or vaccine-associated thrombotic thrombocytopenia (VATT), are rare types of blood clotting syndromes that were initially observed in a number of people who had previously received the Oxford–AstraZeneca COVID‑19 vaccine (AZD1222) during the COVID‑19 pandemic. It was subsequently also described in the Janssen COVID‑19 vaccine (Johnson & Johnson), leading to the suspension of its use until its safety had been reassessed.
In April 2021, AstraZeneca and the European Medicines Agency (EMA) updated their information for healthcare professionals about AZD1222, saying it is "considered plausible" that there is a causal relationship between the vaccination and the occurrence of thrombosis in combination with thrombocytopenia and that, "although such adverse reactions are very rare, they exceeded what would be expected in the general population". AstraZeneca initially denied the link, saying "we do not accept that TTS is caused by the vaccine at a generic level". However, later in legal documents filed in February 2024, AstraZeneca admitted its vaccine 'can, in very rare cases, cause TTS'.
On 5 May 2022, the FDA posted a bulletin limiting the use of the Janssen Vaccine to very specific cases due to further reassessment of the risks of TTS, while also FDA also stating in the same bulletin that the benefits of the vaccine outweigh the risks.
Multiple large cohort studies have demonstrated that thrombotic complications after COVID-19 vaccination are very rare, and occur far less frequently than after SARS-CoV-2 infection. A large study using data from more than 29 million people in England found that the risk of cerebral venous sinus thrombosis (CVST) after COVID-19 infection was roughly 8–10 times higher than after the Oxford–AstraZeneca vaccine, and several-fold higher than after mRNA vaccines.
Another population-level analysis published in The BMJ showed that the incidence of thrombocytopenia and thromboembolic events after vaccination was orders of magnitude lower than after COVID-19 infection, concluding that “the risks of these adverse events are substantially higher following SARS-CoV-2 infection than after vaccination.” Overall, the scientific consensus is that while rare vaccine-associated clotting syndromes can occur, COVID-19 infection poses a far greater thrombotic risk.