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Persistent questions about whether rare but serious blood clots are more common in people receiving the AstraZeneca rush against COVID-19 than in the general population, and what causes them, have continued to undermine confidence in the beleaguered vaccine.

The European Medicines Agency, which has stated that the benefits outweigh the risks so the vaccine should continue to be used, will provide an updated assessment next week.

– What was observed?

The blood clots seen in a handful of people vaccinated with AstraZeneca are described by the French Medicines Agency (ANSM) as “highly atypical”.

“This large vein thrombosis is unusually located in the brain and even less often in the digestive tract,” the agency commented.

It is also linked to a condition characterized by abnormally low blood platelet levels. These are small fragments of cells in our blood that form clots to stop or prevent bleeding.

In mid-March, the German drug authority, the Paul Ehrlich Institute (PEI), was the first national health authority to describe one of them as a deviating number of cases with these rare cerebral blood clots, mainly among younger and medium-sized women.

According to some specialists, these symptoms suggested something called disseminated intravascular coagulation (DIC), in which blood clots form throughout the body.

Odile Launay, a member of the scientific panel advising the French government on COVID-19 vaccines, told AFP that the disease involves “both thrombosis and bleeding” even in extreme cases of sepsis.

– connection with the vaccine?

“A causal link with the vaccine has not been established but it is possible and further analysis is ongoing,” the EMA said last week.

The agency is expected to meet on this issue from April 6-9.

Other specialists were more categorical.

“We need to stop speculating whether or not there is a link – all cases showed these symptoms three to 10 days after inoculation with the AstraZeneca vaccine,” said Pal Andre Holme, leader of a team at Oslo National Hospital working on these Cases works. told Norwegian television.

“We haven’t found any other triggering factor.”

The Norwegian National Medicines Agency supported this assessment. One of its executives, Steinar Madsen, said, “It is likely to be linked to the vaccine.”

For its part, France’s ANSM pointed to “the very unusual type of thrombosis, similar clinical profile and time of onset” and stated that there was a “low” risk.

– How big is the risk?

As of March 31, the EMA identified 62 cases of cerebral venous sinus thrombosis (CVST) worldwide – 44 of them in Europe – among 9.2 million doses of AstraZeneca.

Of these, 14 resulted in death, although it is not possible to definitively attribute deaths to this rare form of thrombosis, EMA head Emer Cooke said in a video conference last week.

The statistics, she added, are comprehensive or close to it.

In Germany, according to the Paul Ehrlich Institute, there were 31 suspected cases of CVST – 19, accompanied by a decrease in blood platelets – with nine deaths.

These cases were spread across 2.8 million AstraZeneca injected vaccine doses, or just over one case per 100,000 doses.

The comparable figures for France are 12 cases and four deaths from 1.9 million doses and for Norway five cases and three deaths from 120,000 doses.

In the UK, where AstraZeneca was given more than any other country, 30 cases were recorded on Saturday, including seven deaths in a total of 18.1 million doses.

But as with all drugs, risks must always be weighed against benefits.

“We would all prefer to have drugs that are 100 percent safe but don’t exist,” Adam Finn, professor of pediatrics at Bristol University, told the London-based Science Media Center last week, commenting on the renewed bans the AstraZeneca vaccine in Germany and elsewhere.

“Right now, COVID-19 is the world’s greatest risk to our lives and livelihoods,” added Finn. “We must continue to focus on the need to prevent millions of people from claiming more lives before they are brought under control. The only effective way to do this is through vaccination.”

The EMA has consistently confirmed this view.

“The benefits of the AstraZeneca vaccine in preventing COVID-19, with the associated risk of hospitalization and death, outweigh the risks of side effects,” it said in a March 31 statement.

– risk factors?

Most cases of brain thrombosis have occurred in people under 65 years of age, but it is impossible to draw any conclusions about age as the vaccine has so far been given mainly to younger populations.

The fact that most of the cases have occurred in women may simply reflect the fact that the health sector – predominantly women – was given priority in vaccination.

“As of now, the review has not identified any specific risk factors such as age, gender or a history of bleeding disorders for these very rare events,” the EMA said.

Regardless, some countries have now suspended the vaccine again after some countries temporarily suspended the AstraZeneca sting in mid-March.

Germany decided last week to ban its use to anyone under the age of 60, while in Canada – like France – the age limit is 55 years. In Sweden the age limit is 65 years.

“We don’t just have one vaccine, we have several,” wrote Sandra Ciesek, Professor of Medical Virology at Goethe University Frankfurt, in the journal Science.

“So it makes sense for me to limit the AstraZeneca vaccine to the elderly.”

Norway and Sweden have taken the more radical step of suspending the AstraZeneca vaccine altogether.

– Possible explanations?

At the moment there are only hypotheses, although the EMA is expected to come up with suggestions next week which are the most likely.

In a study published on March 28, which has not yet been peer-reviewed, German and Austrian researchers pointed to a known biological mechanism as a possible explanation for the apparent increase in atypical thrombosis.

The AstraZeneca vaccine, they wrote, is associated with a thrombotic disorder “clinically similar to heparin-induced thrombocytopenia (HIT)”.

HIT is a rare and serious immune system reaction to the anticoagulant heparin.

The authors, led by Andreas Greinacher from the University of Greifswald, suggested a name for what they termed a new syndrome: “Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT)”.

Researchers at Oslo National Hospital had previously suspected that cases could have been triggered by a “strong immune response” to the vaccine.

An association of French scientists and doctors called On the Side of Science has said that such an immune response could be caused by accidentally inserting the needle into a vein in the upper arm rather than by muscle.

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