Questions mount on AstraZeneca vaccine’s ties to rare blood clots

Though very uncommon, the unique type of thromboses seen in dozens who got the shot has experts paying attention; some are urging caution, while others say tiny risk is worth it

A file photo taken on March 11, 2021 shows a vial of the AstraZeneca/Oxford COVID-19 vaccine in Paris on March 11, 2021 (JOEL SAGET / AFP)
A file photo taken on March 11, 2021 shows a vial of the AstraZeneca/Oxford COVID-19 vaccine in Paris on March 11, 2021 (JOEL SAGET / AFP)

How to explain the serious, though rare, blood problems observed in some people vaccinated against COVID-19 with the AstraZeneca shot?

The troubling question remains unanswered before another meeting of the European Medicines Agency (EMA) next week.

The vaccine is not being used in Israel, which has been relying on the Pfizer-BioNTech vaccine.

But it is causing rising concern in Europe and elsewhere.

What has been observed?

The problems observed in some people vaccinated with AstraZeneca are not regular thromboses (formation of blood clots), as initially reported, but a “very atypical” phenomenon, the French Medicines Agency has said.

It is about “thromboses of the large veins, atypical by their localization (cerebral in the majority, but also digestive) and usually associated with thrombocytopenia (a deficit in blood platelets) or with disorders of coagulation,” according to the ANSM.

File: Britain’s Prime Minister Boris Johnson holds a vial of the AstraZeneca COVID-19 vaccine during a visit to a vaccination center at the Health and Well-being Centre in Orpington, south-east London, Monday, Feb. 15, 2021. (Jeremy Selwyn/Pool Photo via AP)

In mid-March, the Paul-Ehrlich Medical Institute (PEI), which advises the German government, was the first to point out “a striking accumulation of a specific form of very rare cerebral venous thrombosis, linked to a deficiency of blood platelets.”

According to specialists, this very specific occurence is reminiscent of a phenomenon called disseminated intravascular coagulation (DIC).

These are “quite exceptional syndromes, which are seen in the context of serious sepsis” (severe infections) and can result “in both thrombosis and hemorrhaging,” infectious disease specialist Odile Launay, a member of the French government’s COVID-19 vaccine committee, told AFP.

What is the link to the vaccine?

“No causal link has been proven, but it is possible and additional analyses are underway,” the European Medicines Agency (EMA) said Wednesday. It will convene again to review the matter on April 6-9.

But other specialists across Europe are more adamant.

“We have to stop speculating whether there is a link or not. All of these cases had these symptoms three to ten days after the AstraZeneca inoculation. We did not find any other triggering factor,” Pal Andre Holme, head of a team reviewing the cases at Oslo’s National Hospital, told Norwegian channel TV2 on March 27.

Ukrainian President Volodymyr Zelensky receives a dose of the AstraZeneca vaccine, marketed under the name CoviShield, as he visits the war-hit Luhansk region, eastern Ukraine, March 2, 2021. (Ukrainian Presidential Press Office via AP)

“The Norwegian Medicines Agency believes there is a probable link with the vaccine,” Steinar Madsen, one of its officials, told AFP.

For its part, the ANSM confirmed on March 26 the existence of a “rare” risk, based on “the very atypical nature of these thromboses, their close clinical pictures and the homogeneous time to onset.”

What is the risk?

This is the essential question.

According to the figures that the EMA published on Wednesday, there are at this stage 62 cases of cerebral venous thrombosis in the world, including 44 in the 30 countries of the European Economic Area (EU, Iceland, Norway, Liechtenstein), of some 9.2 million doses of vaccine administered.

Fourteen deaths have been recorded, without always being reliably attributable to the thromboses, EMA chief Emer Cooke said Wednesday.

However, the agency clarified that the data was only partial.

In Germany, 31 suspected cases of cerebral venous thrombosis (19 of which were accompanied by a drop in blood platelets) have been recorded so far, with 9 deaths, according to the Paul-Ehrlich Institute. This represents a proportion of 1 case per 100,000 doses of AstraZeneca vaccine injected (2.8 million).

Cases have also been reported in France (12, including 4 deaths, out of 1.9 million injections), in Norway (5 cases, including 3 deaths, out of 120,000 injections) and in the Netherlands.

Illustrative: A blood clot blocks red blood cells in a blood vessel (libre de droit; iStock by Getty Images)

In the United Kingdom, which has widely vaccinated with AstraZeneca, a report released on Saturday cited 30 cases and 7 deaths, out of a total of 18.1 million doses administered.

But as with any drug, knowing the risk is not enough: it must be compared with the benefits provided by the product.

“The benefits of the AstraZeneca vaccine in preventing COVID-19, which leads to hospitalizations and deaths, outweigh the risks of side effects,” the EMA insisted on Wednesday.

Risk factors?

So far, most cases have been in people under 65, mostly women. But few conclusions can be drawn from this, as it may come from the fact that the AztraZeneca vaccine was initially mainly used on younger populations.

“For the moment, the examinations have not identified specific risk factors for these very rare events such as age, gender or a medical history including blood clot problems,” according to the EMA.

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