There’s No Scientific Evidence Showing That the AstraZeneca Vaccine Causes Blood Clots

Scientists with the World Health Organization urge nations to heed the data

Photo: Dan Kitwood/Getty Images

More than a dozen countries have suspended the use of the AstraZeneca vaccine, citing concerns that people who received doses from two batches of the vaccine developed blood clots as a result. On Monday, the World Health Organization urged nations to continue using the vaccine, emphasizing that there is no scientific evidence for a causal link between getting the AstraZeneca shot and blood clots.

“As of today, there is no evidence that the incidents are caused by the vaccine and it is important that vaccination campaigns continue so that we can save lives and stem severe disease from the virus,” WHO spokesperson Christian Lindmeier said, according to Reuters.

So far about 30 incidents of blood clots have been reported out of roughly 5 million people who have been vaccinated with AstraZeneca’s two-dose vaccine. Some of these incidents have included “unusual features such as low number of platelets,” noted the European Medicines Agency in a Monday report, but it emphasized that thousands of people in the EU get blood clots every year for a variety of reasons.

The WHO, the EMA, the U.K. Medicines and Healthcare products Regulatory Agency (MHRA), and other health authorities are investigating these concerns. On Friday, WHO Director-General Tedros Ghebreyesus said that “the vaccine can continue to be used while its investigation is ongoing.”

Statistician David Spiegelhalter, chair of the Winton Centre for Risk and Evidence Communication at Cambridge, wrote a very helpful article in The Guardian explaining why the anecdotal reports of people who got blood clots (known as thromboembolic events) after getting the AstraZeneca shot are not evidence for a causal link and, as such, are not cause for concern. Here’s a key passage:

So when the European Medicines Agency says there have been 30 “thromboembolic events” after around 5m vaccinations, the crucial question to ask is: how many would be expected anyway, in the normal run of things?

We can try a quick back-of-the-envelope calculation. Deep vein thromboses (DVTs) happen to around one person per 1,000 each year, and probably more in the older population being vaccinated. Working on the basis of these figures, out of 5 million people getting vaccinated, we would expect significantly more than 5,000 DVTs a year, or at least 100 every week. So it is not at all surprising that there have been 30 reports.

It is surprising that many European nations, as well as Indonesia, have been so quick to suspend the use of AstraZeneca’s vaccine as a precautionary measure in light of this clear line of reasoning. As journalist Matthew Harper writes in STAT, these decisions could make it harder to convince people to get the vaccine, even if, as scientists expect, the concerns turn out to be a false alarm.

In addition, he notes, the decisions “put extraordinary pressure on a large clinical trial of the AstraZeneca vaccine being conducted in the United States, which has not authorized the vaccine’s use. And they raise questions about the rollout of a product that, globally, was expected to be produced most inexpensively and distributed most broadly.” The AstraZeneca vaccine has been distributed widely through Europe, Asia, and to low- and middle-income countries via the COVAX program.

The U.S. Food and Drug Administration has not yet allowed the AstraZeneca vaccine to be used in the United States, but according to a report from Reuters on Friday, the company will seek authorization from the FDA this month or in early April. Data from its 32,000-person U.S. Phase 3 trial, which Harper referred to, are expected to be available in the next few weeks and will provide additional information on the risk of blood clots due to vaccination.

In the meantime, it is important to remember that people who are not vaccinated risk severe illness or death due to Covid-19. So far 2.6 million people around the world have died from Covid-19, noted Ghebreyesus in his address, while no deaths have been found to have been caused by any of the 335 million Covid-19 vaccine doses administered so far.



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Yasmin Tayag

Yasmin Tayag


Editor, Medium Coronavirus Blog. Senior editor at Future Human by OneZero. Previously: science at Inverse, genetics at NYU.