How to explain the serious – and rare – blood problems observed in some people vaccinated against Covid-19 with AstraZeneca? We are starting to get some answers after the announcements from the European Medicines Agency (EMA) on Wednesday.
– What did we observe?
After a further meeting of its experts, the EMA recognized for the first time on Wednesday that the AstraZeneca vaccine could cause very rare blood problems in some people. Until now, she had not established a causal link between the two.
According to her, this “possible link” justifies the risk being mentioned in the vaccine leaflet. The WHO (World Health Organization) remained more cautious, later estimating that the link was “plausible but not confirmed”.
These undesirable effects are not banal thrombosis (formation of blood clots), as initially reported, but a “very atypical” phenomenon, in the words of the French Medicines Agency (ANSM).
These are “very rare cases of blood clots combined with low levels of blood platelets,” the EMA explained on Wednesday.
These thromboses, which occurred “within two weeks after vaccination”, affect “veins of the brain (thrombosis of the cerebral venous sinuses) and the abdomen”, according to the EMA.
They are sometimes accompanied by bleeding, which may seem paradoxical when it comes to blood clots.
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 platelets blood “.
– What’s the risk?
This is the essential question.
According to the latest figures from the EMA, on April 4, there were 222 cases of these atypical thromboses out of 34 million injections carried out with AstraZeneca in the European Economic Area (EU, Iceland, Norway, Liechtenstein) and the Kingdom -United. This resulted in 18 deaths.
In Germany, the country where the alert came from, 31 suspected cases of cerebral venous thrombosis (19 of which were accompanied by a fall in blood platelets) were listed at this stage, with 9 deaths, according to the Paul Institute. -Ehrlich. This represents a proportion of 1 case per 100,000 injected doses (2.8 million).
In the United Kingdom, where we have widely vaccinated with AstraZeneca, there are 79 cases and 19 deaths, out of more than 20 million doses administered, according to a report released on Wednesday by the British drug regulator, the MHRA, alongside the announcements. of the EMA.
Cases have also been reported in France (12, including 4 deaths, out of 1.9 million injections according to the ANSM), in Norway (5 cases, including 3 deaths, out of 120,000 injections) or in the Netherlands.
But as with any drug, knowing the risk is not enough: it must be compared with the benefits provided by the product. This is called the benefit-risk balance.
“Covid-19 carries a risk of hospitalization and death. The blood clot / low platelet combination that has been reported is very rare, and the overall benefits of the vaccine in preventing Covid-19 outweigh on the risks of side effects, “insisted the EMA on Wednesday, as it has done several times since the first alerts.
– Risk factors?
For the moment, most of the cases concern “women under 60 years”, underlined the EMA.
But no conclusion can be drawn from this, because it may come from the fact that this vaccine was initially mainly used on young populations.
In addition, the fact that there are mainly women among the reported cases may be linked to the fact that this vaccine has been injected a lot into health professionals, a priority category. However, these professions are very feminized.
“According to the elements currently available, we have not identified a specific risk factor,” commented the EMA.
In the UK, where the population categories vaccinated with AstraZeneca were wider than elsewhere, cases involve 51 women and 28 men aged 18 to 79.
– An age limit?
After a first wave of suspension in mid-March, several countries have decided to no longer administer this vaccine below a certain age: 30 years for the United Kingdom, 55 years for France, Belgium and Canada, 60 years for Germany and the Netherlands, or 65 for Sweden and Finland.
“We don’t have just one vaccine, we have several. That’s why reserving AstraZeneca for older people seems to make sense to me,” commented a virologist from Goethe University in Frankfurt, Sandra. Ciesek, in Science magazine.
Here again, this reasoning is based on the risk-benefit balance, which varies according to age: the older we get, the more we have a risk of developing a serious form of Covid, and the more it is in our interest to be vaccinated with AstraZeneca, despite the risk of side effects.
British health authorities released a comparative table on Wednesday to support this reasoning. It compares on the one hand the risk of admission to intensive care and on the other hand the risk of side effects caused by the vaccine, according to age.
According to this table, when the virus is circulating strongly, the risk caused by the Covid is 6 times greater than that caused by the vaccine in the 20-29 age group. But it becomes 600 times more important when we move to the 60-69 age group.
Norway and Denmark have made a more drastic choice than just age limits, by stopping the AstraZeneca vaccine altogether for now.
– What reasons?
Although we are not sure, several elements support an abnormal and powerful immune response elicited by the vaccine.
In a study published online on March 28, but not yet evaluated by other scientists, German and Austrian researchers made the connection with another already known mechanism.
The phenomenon associated with the AstraZeneca vaccine “clinically resembles heparin-induced thrombocytopenia (HIT)”, write the researchers led by Andreas Greinacher (University of Greifswald).
HIT is a serious and rare immune reaction triggered in some patients by the anticoagulant drug heparin.
This is “a plausible explanation,” the EMA commented on Wednesday, calling for further studies.
German and Austrian researchers even propose to give a name to the phenomenon observed with the AstraZeneca vaccine (the English acronym VIPIT).
Other French researchers and doctors, united in the collective Du Côté de la Science, put forward an original hypothesis.
They believe that the starting point may be “the accidental injection of the vaccine into a vein within the deltoid muscle” (of the shoulder), which would trigger a “jarring immune response”.