AstraZeneca and Johnson & Johnson: adenoviruses at the center of suspicion - France 24

Paris (AFP)

After AstraZeneca, Johnson & Johnson: These two Covid-19 vaccines, based on the same technology, are suspected of causing a very rare type of blood clots.

Update on what we know.

- What did we observe?

In the case of AstraZeneca and Johnson & Johnson, suspicion arose after cases of thrombosis (formation of blood clots) in a few vaccinated people.

These are not common thromboses, such as simple phlebitis, but very unusual thromboses.

On the one hand, they are atypical by their location: they affect "the veins of the brain (thrombosis of the cerebral venous sinuses)" and, to a lesser extent, the abdomen, the European Medicines Agency said on April 7. (EMA) about AstraZeneca.

Ditto for Johnson & Johnson, with which it is also "thrombosis of the cerebral venous sinuses" which were observed, underlined Tuesday the American health authorities, FDA and CDC, which suspended the use of this vaccine in the United States.

In addition to their location, these thromboses are intriguing because they are accompanied by a drop in the level of blood platelets, the cells that help the blood to clot.

Paradoxically, this can therefore cause hemorrhages in addition to blood clots.

"The treatment for this particular type of blood clot is different from what would normally be given," warns the FDA and the CDC.

On April 7, the EMA recognized for the first time that these very rare blood problems could be caused by the AstraZeneca vaccine.

The two vaccines are "the probable cause" of these very atypical events, for his part estimated Tuesday an official of the American FDA, Peter Marks.

Johnson & Johnson's vaccine is authorized in the European Union (under the name Janssen) but not yet administered.

AstraZeneca's is used in the EU, but not yet licensed in the US.

- What reasons?

Although nothing is proven yet, it is increasingly likely that these blood problems are related to the technique on which these two vaccines are based.

Both are so-called "viral vector" vaccines: we take another virus as a carrier, which we modify so that it carries genetic information in the body to fight Covid.

And both use a very common type of virus called adenovirus as a carrier.

AstraZeneca opted for a chimpanzee adenovirus, Johnson & Johnson for a human adenovirus.

The fact that similar problems have been observed with these vaccines "could suggest that the problem is with the adenovirus vector," said David Fisman, epidemiologist at the University of Toronto, on Twitter.

"Everything suggests that it is linked to the adenovirus vector", added, also on Twitter, Mathieu Molimard, French specialist in pharmacology.

Indeed, "these cases do not exist to date with RNA vaccines", those of Pfizer / BioNTech and Moderna, which use another technique, messenger RNA.

It remains to be seen whether this type of blood problem is also observed with the Russian vaccine Sputnik V, which also uses two adenoviruses as vectors.

It is authorized in about sixty countries for the moment, but not in the European Union or in the United States.

- What mechanisms?

Specialists favor an abnormal and powerful immune response caused by these vaccines.

In a study published online on March 28 and on AstraZeneca, German and Austrian researchers made the connection with another already known mechanism.

The observed phenomenon "clinically resembles heparin-induced thrombocytopenia (HIT)", write these 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 April 7, 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).

- What's the risk?

This is the essential question.

In the case of AstraZeneca, on April 4, there were 222 cases of these atypical thromboses out of 34 million injections carried out in the European Economic Area (EU, Iceland, Norway, Liechtenstein) and the United Kingdom, according to the EMA.

This resulted in 18 deaths (as of March 22).

The thromboses occurred "within two weeks of vaccination," according to the EMA.

In the case of Johnson & Johnson, US authorities have documented six cases (including one death) out of over 6.8 million doses administered in the United States, "and symptoms occurred 6 to 13 days after vaccination." , according to the FDA and the CDC.

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 April 7 about AstraZeneca.

- Risk factors?

In both cases, young women seem particularly concerned.

Most of the cases seen with AstraZeneca involve "women under the age of 60," according to the EMA.

And the six cases identified in the United States in connection with Johnson & Johnson are "women aged 18 to 48".

But it is too early to draw a conclusion.

"According to the elements currently available, no specific risk factor has been identified," the EMA commented on AstraZeneca.

However, after a first wave of suspension in mid-March, several countries have decided to no longer administer the AstraZeneca 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 years 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 have released a comparative table 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 and over a period of 16 weeks.

According to this table, when the virus is circulating strongly, the risk caused by the Covid is six 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.

© 2021 AFP