Embolic and thrombotic events after COVID-19 vaccination

[14] 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".

[21][22] Cerebral venous sinus thrombosis may cause severe headache, stroke-like symptoms (weakness of a limb and/or facial muscles), seizures and coma.

[30] Thrombocytopenia is generally a common symptom after or during many viral infections,[31] and it "has been consistently reported" after intravenous administration of adenoviral gene transfer vectors,[32] although its mechanisms are not yet clear.

If the platelet count is decreased, determination of the D-dimer and fibrinogen levels may be performed, with hematology expert advice recommended if these are elevated above specific cut offs.

[34] UK guidelines by the British Society for Haematology recommend the administration of intravenous immunoglobulin (IVIG) to reduce levels of the pathogenic antibody.

[34] The Paul Ehrlich Institute has recorded 31 cerebral venous sinus thromboses (CVST) and nine deaths out of 2.7 million vaccinated in Germany with the AZD1222.

However, because Germany had previously restricted AZD1222 to under 65s, the population vaccinated there with AZD1222 is comparatively younger, and consequently contained a higher proportion of women taking the contraceptive pill.

[36] The UK Medicines and Healthcare products Regulatory Agency (MHRA) reporting regards AZD1222 to 3 November 2021, recording 73 deaths out of 425 cases (17%) in the context of 24.9 million first doses administered.

[12] Accordingly, the EMA advised that people who received the vaccine and experienced symptoms suggestive of thrombosis, including shortness of breath, blurred vision and severe or persistent headache, should seek medical attention.

[12] In a press briefing on 7 April 2021,[40][41] Emer Cooke, the executive director of the European Medicines Agency (EMA) began by stating "Our safety committee, the Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency, has confirmed that the benefits of the AstraZeneca vaccine in preventing COVID‑19 overall outweigh the risks of side effects.

[44] The UK Medicines and Healthcare products Regulatory Agency (MHRA) held a news conference on 7 April 2021, and while there is no proof that the AZD1222 vaccination caused the rare blood clots, they indicated the possibility of a link is getting stronger.

[45] Also on 7 April 2021, an interim statement from the WHO said its advisory body, GACVS, found any "causal relationship" between the rare blood clot cases and AZD1222 to be "plausible but is not confirmed".

[54] In its safety update of 29 March 2021, the EMA indicated it had initiated investigations into the very rare cases of specific embolic and thrombotic events in combination with thrombocytopenia (low levels of blood platelets) and related bleeding including disseminated intravascular coagulation and cerebral venous sinus thrombosis (CVST), noting any link with AZD1222 is not proven but could not be excluded.

[12] The EMA also initiated an assessment for all COVID‑19 vaccines used in the EU for immune thrombocytopenia (ITP), described as low blood platelet levels that could lead to bruising and bleeding, as a possible side effect, whilst also stating that up to this point no link with any COVID‑19 had been established.

[59] A study convened by a group of British hematologists on 19 March 2021, just two days after the acknowledgement of the condition, published its finding in The New England Journal of Medicine, establishing case definition criteria.

The study included 294 participants who presented with symptoms of thrombocytopenia and thrombosis after receipt of the first dose of the Oxford–AstraZeneca COVID‑19 vaccine, showing an independent association between baseline platelet count and the presence of intracranial hemorrhage.

The study showed an overall mortality rate of 22% and set out plans for additional research to determine the genetic factors that may increase risk of the condition and identify potential therapeutic agents.