Blood Clot Risks 'Lower With COVID Vaccines Than From Infection'

Peter Russell

August 27, 2021

Vaccination with both the AstraZeneca/Oxford vaccine or the Pfizer BioNTech vaccine increases short-term risk of some haematological and vascular adverse events leading to hospitalisation or death, a study has suggested.

The research, published in The British Medical Journal , showed an increased risk of thrombocytopenia and venous thromboembolism with the AstraZeneca vaccine, and an increased risk of arterial thromboembolism following the Pfizer vaccine.

However, the risk of those conditions following a first dose of either vaccine were significantly lower than the same outcomes from infection with the SARS-CoV-2 virus, researchers stressed.

Large Scale Analysis

They said their analysis was the first to compare the AstraZeneca and Pfizer vaccines with those risks from COVID on a large scale.

The study drew on data from 29.1 million people who were vaccinated between December 1, 2020 and April 24, 2021. During that time, 19.6 million people received a first dose of AstraZeneca, 9.5 million had the Pfizer jab, and 1.8 million tested positive for SARS-CoV-2.

The research team, led by the University of Oxford, found an increased risk of thrombocytopenia and venous thromboembolism 8 to 14 days after a first dose of the AstraZeneca vaccine, but no increased risk of arterial thromboembolism.

Conversely, the Pfizer vaccination was associated with arterial thromboembolism 15 to 21 days after vaccination, but it was not associated with increased relative risk of thrombocytopenia or venous thromboembolism.

The researchers estimated the number of exposures needed for one excess event, and the excess number of events per 10 million exposed to each vaccine, or with a SARS-CoV-2 positive test

Specifically, with AstraZeneca vaccine, there were:

  • 107 excess events for thrombocytopenia

  • 66 for venous thromboembolism

  • 7 for CVST (cerebral venous sinus thrombosis)

With the Pfizer vaccine there was an estimated 143 extra cases of ischaemic stroke.

In context, for SARS-Cov-2 infection, there was an estimated:

  • 934 extra cases of thrombocytopenia

  • 12,614 cases of venous thromboembolism

  • 1699 cases of ischaemic stroke

  • 20 cases of CVST

Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford, who led the study, said people should "be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2".

Aziz Sheikh, professor of primary care research and development at the University of Edinburgh, who co-authored the paper, said the analysis "clearly underscores the importance of getting vaccinated to reduce the risk of these clotting and bleeding outcomes in individuals, and because of the substantial public health benefit that COVID-19 vaccinations offer".

The researchers said their study had been limited by having to restrict their analysis to first doses only of a COVID vaccine.

'Important' Research

Dr Peter English, a retired consultant in communicable disease control, described the paper as "very important".

He told the Science Media Centre: "The fact that such adverse events were identified in vaccine recipients, despite being very uncommon, is testimony to the high quality of our vaccine adverse event surveillance systems."

He added that the "rarity of these adverse events makes it difficult to quantify precisely their frequency after specific vaccines".

Dr Richard Francis, head of research at the Stroke Association, said: "The risk of all types of stroke for all groups of people is lower after taking the vaccine; this means that the benefits of the vaccine for stroke risk alone, is greater than the risk. 

"Knowing that the vaccine could save you from the deadly or severely disabling effects of stroke, as well as protect you from getting a nasty bout of COVID-19 should be enough to persuade anyone to get vaccinated."

BMJ 2021;374:n1931

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