This Is What We Know So Far About AstraZeneca’s COVID-19 Vaccine And Blood Clots

So far there’s no clear evidence of a safety problem. But several European countries have paused vaccinations with AstraZeneca’s shots while investigations continue.

Germany, France, Italy, and Spain have all halted vaccinations with AstraZeneca’s COVID-19 vaccine, following reports of rare blood clots in people given the shots.

These moves by the four largest countries in the European Union follow pauses in several smaller member nations — and mark a new crisis in confidence for an affordable vaccine that is seen as the best hope of accelerating COVID-19 vaccination across the developing world.

Experts stress that there is still no clear evidence that the vaccine is causing these reported clots, and the cases are being investigated by regulators. But the latest concerns follow earlier questions from scientists over how the company has communicated issues with the safety and efficacy of its vaccine. It also comes shortly before AstraZeneca is expected to release results from a large-scale clinical trial that will determine whether the FDA authorizes the vaccine for use in the US.

The pauses to vaccination in Europe were triggered by reports of three hospitalizations, including one patient who died, among people given the vaccine in Norway, and a further death in Denmark. In a statement issued on March 11, the European Medicines Agency — the regulator that is the EU’s equivalent of the FDA — said there had been a total of 30 cases of “thromboembolic events,” or blood clots, among the 5 million people who have received AstraZeneca shots in the EU.

“There is currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine,” the EMA said. “The position of EMA’s safety committee … is that the vaccine’s benefits continue to outweigh its risks and the vaccine can continue to be administered.”

AstraZeneca said that there had been a total of 37 reported cases of pulmonary embolism and deep vein thrombosis, both conditions caused by blood clotting, which is fewer than would be expected from the rate in the general population.

“Around 17 million people in the EU and UK have now received our vaccine, and the number of cases of blood clots reported in this group is lower than the hundreds of cases that would be expected,” the company’s chief medical officer Ann Taylor said in a statement issued on March 14.

And the World Health Organization is urging countries to keep using the vaccine, echoing that there is no evidence that it causes blood clots. WHO experts are meeting to discuss the reports of blood clots on Tuesday.

However, a handful of incidents are giving some countries pause. According to German health minister Jens Spahn, there have been seven reported cases of a rare condition, cerebral venous sinus thrombosis (CVST), out of the 1.6 million people given the AstraZeneca vaccine in Germany. The Paul Ehrlich Institute, part of the German health ministry, said Monday that it was concerned that the condition was reported alongside unusual bleeding and a low count of blood platelets, which form clots and prevent bleeding.

The handful of cases seen in Germany so far would be more than expected, but could be a statistical fluke. “[T]he probability of such a statistical clustering for one outcome is actually quite high even when no cause and effect exists,” said Paul Hunter, specialist in public health at the University of East Anglia, in a comment distributed through the UK’s Science Media Centre.

The EMA is meeting to discuss the concerns about blood clotting on Tuesday and is expected to release its conclusions on Thursday. A prolonged hiatus in vaccination would be a worrying prospect, given that COVID-19 cases are rising again in several EU nations — with Italy experiencing a particularly alarming surge — as more contagious variants spread.

Experts contacted by BuzzFeed News said that, without knowing more about the patients’ cases, there wasn’t enough evidence to draw clear conclusions about whether the vaccine caused clotting. “We would want to know the age of the cases, their prior medical history, and conditions that might predispose their risk for either a clot or a bleed,” said Orly Vardeny of the University of Minnesota, a pharmacist who specializes in cardiac health.

“It may or not be vaccine-related. If it is vaccine-related, and if it’s not being seen everywhere, the most likely explanation would be a production batch issue,” John Moore, a virologist at Weill Cornell Medical College in New York who works on vaccine development, told BuzzFeed News. (Some countries, including Austria, have specifically halted vaccination from particular batches of the AstraZeneca vaccine.)

The new concerns are the latest in a series of stumbles for AstraZeneca’s COVID-19 vaccine, which was developed at the University of Oxford. Back in November, AstraZeneca claimed in a press release that its vaccine was 70% effective overall, and could be up to 90% effective if given in a half dose for the first of two shots. But many scientists were skeptical, especially after the drug company admitted that this regime was the result of a dosing miscalculation. AstraZeneca later added to the confusion by changing its explanation for the claimed 90% efficacy from the dosing itself to the lag between the two doses.

A more serious blow came in February, when trials in South Africa showed the vaccine wasn’t effective against the more contagious variant of the coronavirus circulating there. South Africa abruptly halted plans to use AstraZeneca’s vaccine and switched instead to vaccines made by Johnson & Johnson and Pfizer/BioNTech.

Now scientists are waiting for the results of a large trial of the AstraZeneca vaccine conducted in the US and other countries that will be the cornerstone of the company’s application to have the vaccine authorized for use by the FDA. But some experts wonder whether adding AstraZeneca’s to the armory of vaccines being used in the US will create more problems, with anti-vaxxers likely to seize on the negative publicity surrounding it to undermine the wider vaccination drive.

“Does it really have a niche to fill? Would it be trusted enough? Or is it going to be just another headache?” said Moore.

Children’s Health Defense, an organization that has peddled misinformation and conspiracy theories about vaccines for years, has already highlighted the news that several European countries are pausing their use of the vaccine.

Where the vaccine could make a huge difference is in the developing world, especially in Africa, where vaccine rollout is lagging far behind. The Biden Administration is already under pressure to donate already-manufactured doses of the AstraZeneca vaccine to poorer nations, the New York Times reported on March 11. Meanwhile, COVAX — the collaboration formed by the WHO, the Coalition for Epidemic Preparedness Innovations, and Gavi, the Vaccine Alliance, to provide affordable vaccines to poorer countries — is relying on using AstraZeneca’s vaccine to deliver hundreds of millions of doses across the developing world.

“Safety is our paramount concern: We know that national authorities and the WHO are monitoring the situation closely and the COVAX Facility will be following their guidance and recommendation,” a spokesperson for Gavi said in response to queries from BuzzFeed News. “Currently no causal link has been established between the vaccine and thromboembolic events in individuals, and the vaccine remains an important and effective public health tool in the fight against this pandemic.”


This story has been updated to clarify that cases of cerebral venous sinus thrombosis were identified in Germany.

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