02 March 2022

The Oxford AstraZeneca vaccine for COVID-19 is associated with a very small cerebral clot risk, according to two recent analyses.

Over the past year, thromboses have been found in sites such as the cerebral veins following COVID vaccination. This “vaccine-induced immune thrombotic thrombocytopenia” is probably due to platelet stimulation encouraging clotting, explain Dr William Whiteley of the University of Edinburgh and colleagues in PLoS Medicine.

In the first study, a team led by Dr Whiteley set out to measure the risk of thrombocytopenia or blood clots at the population level. They analysed electronic health records of 46 million adults in England, comparing venous and arterial events before and after the participants' first COVID jabs.

This showed that in people less than 70 years of age, rates of hospitalisation due to intracranial venous thrombosis or thrombocytopenia were increased after vaccination with the Oxford-AstraZeneca vaccine, but not the Pfizer-BioNTech vaccine.

The authors write, "the absolute increase in the risk of these events was very small" – between 0.9 and 3 additional cases per million, depending on age and sex. They added that the risk is “likely to be outweighed by the vaccines’ effect in reducing COVID-19 mortality and morbidity”.

In people aged 70 or older, the rates of arterial and venous events were in fact lower after vaccination with either vaccine, after adjusting for potential confounding factors.

A second study, published in the same journal, analysed a dataset of 11 million adults in England, Scotland, and Wales.

Dr Steven Kerr, also of the University of Edinburgh, and colleagues point out that as several countries have restricted the Oxford AstraZeneca vaccine to older age groups over safety concerns, "large datasets are required to precisely estimate the association" between COVID vaccination and cerebral venous sinus thrombosis (CVST).

They linked national datasets from primary care, secondary care, mortality, and virological testing, on 11,637,157 people.

Their analysis showed an elevated risk of CVST, equating to 0.25 events per million people during the four weeks after vaccination with the Oxford AstraZeneca vaccine, but not the BioNTech vaccine.

“Public health authorities may wish to take this evidence into account when communicating the benefits and risks associated with COVID-19 vaccines to the general public,” they conclude.

Source: Whiteley WN, Ip S, Cooper JA, Bolton T, Keene S, Walker V, Denholm R, Akbari A, Omigie E, Hollings S, Di Angelantonio E, Denaxas S, Wood A, Sterne JAC, Sudlow C; CVD-COVID-UK consortium.  (2022) “Association of COVID-19 vaccines ChAdOx1 and BNT162b2 with major venous, arterial, or thrombocytopenic events: A population-based cohort study of 46 million adults in England.” PLoS Medicine, doi: 10.1371/journal.pmed.1003926

Link: http://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1003926

Source: Kerr S, Joy M, Torabi F, Bedston S, Akbari A, Agrawal U, Beggs J, Bradley D, Chuter A, Docherty AB, Ford D, Hobbs R, Katikireddi SV, Lowthian E, de Lusignan S, Lyons R, Marple J, McCowan C, McGagh D, McMenamin J, Moore E, Murray JK, Owen RK, Pan J, Ritchie L, Shah SA, Shi T, Stock S, Tsang RSM, Vasileiou E, Woolhouse M, Simpson CR, Robertson C, Sheikh A. (2022) “First dose ChAdOx1 and BNT162b2 COVID-19 vaccinations and cerebral venous sinus thrombosis: A pooled self-controlled case series study of 11.6 million individuals in England, Scotland, and Wales.” PLoS Medicine, doi: 10.1371/journal.pmed.1003927

Link: http://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1003927

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