AstraZeneca vaccine safe, but rare risk of blood clots not ruled out, EU regulator says

A vaccine shown in front of AstraZeneca and University of Oxford signs.
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Editor's note: On April 7, the EMA confirmed that the AstraZeneca vaccine can sometimes cause blood clots and low platelet counts, although the exact mechanism is still unknown. 

The COVID-19 vaccine made by AstraZeneca and the University of Oxford is safe and not linked to an increased risk of blood clots, according to an investigation by the European Union's European Medicines Agency (EMA).

"This is a safe and effective vaccine. Its benefits in protecting people from COVID-19 with the associated risks of death and hospitalization outweigh the possible risks," Emer Cooke, the EMA's executive director, said at a news conference Thursday (March 18). 

However, the EMA investigation uncovered a few "rare and unusual but very serious clotting disorders" that the committee probed more closely, Cooke said. These included conditions such as disseminated intravascular coagulation (DIC), in which clots form extensively throughout the body, and cerebral venous thrombosis (CVT), a rare form of stroke. 

"As of last night, seven cases of this disseminated intravascular coagulation and 18 cases of cerebral venous sinus thrombosis were reported out of almost 20 million vaccinated people," Cooke said. "After days of in-depth analysis of lab results, clinical reports, autopsy reports and other information from the clinical trials, we still cannot rule out definitively a link between these cases and the vaccine," she said. 

Even if there is a link, if every one of these cases was caused by the vaccine, this would translate to a rate of roughly 1.25 per million vaccinated people. 

The committee will launch further investigations of these extremely rare clotting events and will continue to monitor any additional cases that crop up. In the meantime, they concluded that the vaccine is safe and effective enough to warrant its continued use in the general public.

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The EMA initially launched its investigation after many European countries — including Germany, France, Italy and Spain — halted their distribution of the AstraZeneca vaccine, citing concerns about blood clots and abnormal bleeding among vaccinated people, CNBC reported. In response to these reports from Europe, Indonesia also decided to delay its rollout of the vaccine, and Thailand briefly suspended use of the shots last weekend, The Associated Press reported

More than 17 million people in the U.K. and EU have already received the AstraZeneca shot, according to a March 14 statement from AstraZeneca. "The number of cases of blood clots reported in this group is lower than the hundreds of cases that would be expected among the general population," Ann Taylor, AstraZeneca's chief medical officer, said in the statement. 

Furthermore, the company has not observed any evidence of increased bleeding among the tens of thousands of people in clinical trials for the vaccine, according to the statement. 

As of March 8, the company had received 22 reports of pulmonary embolism (blood clots in the arteries of the lungs), and 15 reports of deep vein thrombosis (blood clots in the deep veins of the body), across the EU and U.K. "This is much lower than would be expected to occur naturally in a general population of this size," the company stated. 

The company noted that "available evidence does not confirm that the vaccine is the cause" of these clots. However, the nature of the more unusual clotting disorders, like CVT, drew concern from some doctors. 

For instance, numerous patients showed widespread blood clots, internal bleeding and low platelet counts; platelets are small blood cells involved in clotting. And several people in Germany developed CVT, Science Magazine reported. Germany received seven reports of these rare strokes and found that the events occurred four to 16 days after each patient's vaccination; three of these patients died. This pattern is what prompted Germany to pause use of the shots, Science reported.

In theory, the clotting and bleeding disorders could stem from an overblown immune reaction to the vaccine, Dr. Steinar Madsen, medical director of the Norwegian Medicines Agency, told Science. Future EMA investigations will likely explore this possibility.  

Other scientists have wondered whether the problem may lie with specific batches of the vaccine, which they posit may have become contaminated or contained a higher dose than usual. In general, AstraZeneca supplies European countries with vaccines produced at several different facilities. But the EMA's safety committee, Pharmacovigilance Risk Assessment Committee (PRAC), has "found no evidence of a quality or a batch issue," PRAC Chairwoman Sabine Straus said at the news conference.  

Just because the clots and bleeding disorders emerged after vaccination "does not necessarily mean that the events are linked to vaccination itself, but it is good practice to investigate them," the World Health Organization noted in a statement released March 17. Blood clots are the third-most-common cardiovascular condition globally, and they occur frequently in the general population, according to the statement. 

For instance, an estimated 300,000 to 600,000 U.S. residents develop blood clots in their lungs or deep veins each year, according to the Centers for Disease Control and Prevention (CDC). EU residents develop blood clots at similar annual rates, and in clinical studies of the AstraZeneca vaccine, the rate of clotting episodes did not exceed the expected rate, the EMA reported, according to The Associated Press

But again, rarer conditions such as DIC and CVT will need to be investigated more closely in vaccinated people.

Echoing the EMA's conclusions, the U.K. Medicines and Healthcare products Regulatory Agency (MHRA) also found no link between the vaccine and blood clots, according to BBC News

Those who have received the AstraZeneca vaccine can submit reports to the company about any potential side effects or health-related issues they experience. The U.K. publishes summaries of these reports online, and so far, the most commonly reported symptoms include mild to moderate side effects, like headache, fatigue, soreness at the injection site and chills. Scattered reports of thrombosis, or blood clots, can also be seen in these summaries.  

Clearly, not all of these reported conditions could be caused by the vaccine. People have reported post-vaccine conditions such as "sunburn," "breath odor," "hiccups," "arthropod stings" and "genital herpes," as seen in the summary from early January to late February. One recipient reported getting a face-lift after being vaccinated. In other words, some medical conditions reported post-vaccination may be totally unrelated to the shots. 

Originally published on Live Science. 

Nicoletta Lanese
Channel Editor, Health

Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.