Blood Clots More Likely After Covid-19 Than After Vaccination, Study Finds

Review by Oxford shows risk of rare, serious blood clotting is eight to 10 times greater in people who caught the coronavirus than among people who received any of the first three Western-developed vaccines

A study by the University of Oxford found the risk of rare but sometimes-deadly blood clotting is roughly eight to 10 times greater in Covid-19 sufferers than among people who have received any of the first three Western-developed vaccines widely available.

The study, involving vaccinations from Pfizer Inc. PFE +2.31% and BioNTech SEBNTX +7.60% another from Moderna Inc. MRNA +6.53% and one from AstraZeneca AZN +0.01% PLC, adds to competing evidence related to blood clotting that regulators and governments may need to take into account as they weigh continued deployment of vaccines.

U.S. officials have recommended a pause in the administration of the single-dose Covid-19 vaccine developed by Johnson & Johnson JNJ +0.79% as they study a possible link between the shot and a very small number of instances of severe blood clotting. The Oxford study didn’t look at patient data for recipients of the J&J shot.

The AstraZeneca shot hasn’t been approved for use in the U.S., but many governments across Europe have restricted it, amid several instances of similar blood clotting involving cerebral venous thrombosis, or CVT, or CVST.

AstraZeneca has said it is helping regulators in continuing safety reviews and says the benefits of the vaccine outweigh its risks. J&J said it is aware of the cases involving blood clots and is working with health authorities.

The scientists behind the Oxford study are different from the Oxford scientists who developed the vaccine in partnership with AstraZeneca, the study researchers said Thursday. The study, which hasn’t been peer reviewed, said it showed the clotting-related risks from getting sick with Covid-19 are much greater than any clotting risks presented following administration of the three Western-developed Covid-19 vaccines that are already in wide circulation, and should help assuage vaccine hesitancy broadly. The researchers said CVT could be more common in Covid-infected adults than previously estimated, and they set out to compare the rate with severe blood clotting that appears to be linked to vaccinations.

They cautioned, however, against putting too much stock in the specific findings of the relative risks of Covid-infected and general populations or varying risks among vaccines. They said the data used was too limited for statistically valid comparisons of CVT occurrence among recipients of different vaccines or with the general population. The Oxford scientists said other limits of the data include the rarity of the clotting in all groups and the inclusion of children in the U.S. data set. The data review also didn’t shed light on the cause of clots or mechanisms in the virus or vaccines that might overlap or contribute to clots.

The main finding regarding CVT is that “the risk is many-fold higher after Covid-19 than after receiving a vaccine,” said Masud Husain, an Oxford professor of neurology and cognitive neuroscience who was part of the study.

The Oxford study, which looked at a database of primarily U.S. patient data, found the risk of CVT within two weeks of a Covid-19 infection is roughly 100 times greater than in the general population—again caveated by data and statistical limitations. The study found the risk was eight to 10 times greater than among adults receiving any one of the three vaccines from Pfizer-BioNTech, Moderna and AstraZeneca.

Pfizer and Moderna have said their shots are safe.

Researchers said the limited data used, which lags behind real-world vaccine rollouts, didn’t include anyone who reported taking the J&J shot. The data included 513,000 patients who were diagnosed with Covid-19 from January 2020 through March 25, 2021.

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