Risk of blood clots higher with Covid infection than vaccines, finds large study

Covid infection carries a substantially higher and longer risk of blood clots and other related adverse events than either Oxford/Astra Zeneca or Pfizer/BioNTech vaccination, a large UK study has concluded.

Analysis of 29 million people given a first dose of either vaccine in the first few months of the campaign showed some increased risks of haematological and vascular adverse events for a short time after vaccination.

Astra Zeneca vaccination was found to be associated with increased risk of thrombocytopenia, venous thromboembolism, and other rare arterial thrombotic events.

And there was an increased risk of arterial thromboembolism and ischaemic stroke after a first dose of the Pfizer-BioNTech vaccine, the team report in the BMJ.

A potential increased risk of cerebral venous sinus thrombosis was found after a first dose of both vaccines—a week later with Pfizer than with Astra Zeneca – but the numbers were small and further confirmation is needed, they said.

Their findings have ‘major implications’ for healthcare policy and further research but also need corroboration from other large datasets in other countries, they wrote.

What is clear, they stressed, was that the risk of all these adverse events was much higher after Covid-19 infection.

The initial trials would not have been large enough to pick up these extremely rare adverse events, they noted.

Their results showed:

• increased risk of thrombocytopenia after Oxford/Astra Zeneca vaccination (incidence rate ratio 1.33, 95% confidence interval 1.19 to 1.47 at 8-14 days) and after a positive SARS-CoV-2 test (5.27, 4.34 to 6.40 at 8-14 days)
• increased risk of venous thromboembolism after Oxford/Astra Zeneca vaccination (1.10, 1.02 to 1.18 at 8-14 days) and after SARS-CoV-2 infection (13.86, 12.76 to 15.05 at 8-14 days)
• increased risk of arterial thromboembolism after Pfizer/BioNTech vaccination (1.06, 1.01 to 1.10 at 15-21 days) and after SARS-CoV-2 infection (2.02, 1.82 to 2.24 at 15-21 days)

Study leader Professor Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford, said: ‘People should be aware of these increased risks after Covid-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2.’

She added: ‘This research is important as many other studies, while useful, have been limited by small numbers and potential biases.

‘Electronic healthcare records, which contain detailed recording of vaccinations, infections, outcomes and confounders, have provided us with a rich source of data with which to perform a robust evaluation of these vaccines, and compare to risks associated with Covid-19 infection.’

 Co-author Professor Aziz Sheikh, professor of primary care research and development at the University of Edinburgh said: ‘This enormous study, using data on over 29 million vaccinated people, has shown that there is a very small risk of clotting and other blood disorders following first dose Covid-19 vaccination. 

‘Though serious, the risk of these same outcomes is much higher following SARS-CoV-2 infection.’ 

He added: ‘On balance, this analysis therefore 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.’

Since May, all UK patients aged under 40 have been offered Pfizer or Moderna vaccines instead of Astra Zeneca due to the concerns over rare blood clots.