New Study Finds Slightly Elevated Risk of Bleeding Disorders After AstraZeneca Vaccine

Researchers studying records of 1.7 million adults who received the AstraZeneca AZN 2.29% PLC Covid-19 vaccine in Scotland found a small increased risk of bleeding conditions also sometimes associated with several vaccines routinely given to children, and usually treatable.

The blood disorders are different from a very rare but sometimes deadly blood-clotting condition that—coupled with low platelet levels—researchers in Europe have already linked to the AstraZeneca vaccine. That possible side effect has been the focus of regulatory and government scrutiny in recent months.

In the new paper, published Wednesday in the journal Nature Medicine, researchers from the U.K. and New Zealand said their findings shouldn’t change policies in the U.K. and elsewhere to continue deploying the AstraZeneca shot as a generally safe and effective tool against Covid-19. AstraZeneca and regulators have said they are studying the blood-clotting matter further and that the benefits of the shot outweigh risks for most people. AstraZeneca said Wednesday that more than 500 million doses of the vaccine have helped save more than 100,000 lives, and that its safety is paramount.

The new analysis concludes that the benefits of the AstraZeneca vaccine, along with heightened risks of internal bleeding and blood clots from Covid-19 itself, far outweighed any increased risks from the shot for most people. Because incidents of blood clotting have occurred more often in younger adults, some health officials have said that they should be offered an alternative to the AstraZeneca shot.

The vaccine was developed with the University of Oxford. The new paper, which hasn’t been peer reviewed, provides a deeper dive into a large number of vaccine recipients in a single country. Because these bleeding conditions have been associated with other, long-used vaccines, researchers decided to study potential occurrences in Covid-19 vaccines.

Researchers said the findings show the need for countries to closely monitor vaccine rollouts to help standardize definitions of possible side effects and flag problems that might take months or years to surface.

The study also examined records of around 800,000 people who received the Pfizer Inc. PFE 2.47% vaccine, and found no additional risk of blood clotting or hemorrhaging.

The bleeding risks highlighted in the findings are caused by low platelet levels brought on by a condition called immune-induced thrombocytopenia, or ITP. Platelets are blood cells that promote clotting and prevent bleeding. ITP is rare but has been seen for years after other common vaccines, including for hepatitis B, measles, mumps and rubella, the researchers said. They and other scientists not involved in the study said the condition often goes undiagnosed and rarely causes death.

​Europe’s top drug regulator endorsed AstraZeneca’s vaccine after it was suspended in several countries over blood-clot concerns. WSJ explains what’s at stake for a shot that’s been widely used around the world and may soon be considered for emergency use in the U.S. Photo: Mykola Tys/SOPA Images

Normally the U.K. sees ITP in roughly six to nine people out of 100,000, researchers said Wednesday. Covid-19 has increased that incidence, making a baseline number difficult to pinpoint in the pandemic, they said. In the study sample, an additional estimated 1.13 people out of 100,000 experienced the low-platelet condition after one dose of the AstraZeneca shot.

In total, the Nature study examined records of 2.5 million people who received their first of two scheduled Covid-19 vaccine doses in Scotland between Dec. 8 and April 14.

In an accompanying Nature editorial, scientists not involved in the study said immune-induced thrombocytopenia is often difficult to diagnose. It is also challenging to separate pre-existing conditions from the effects of vaccines, they said. “The risk of vaccination-induced ITP at the rate proposed seems to be far lower than the many risks associated with Covid-19 itself,” they wrote.

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Write to Jenny Strasburg at

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Appeared in the June 10, 2021, print edition as ‘New Small Risk Seen In AstraZeneca Shot.’


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