Janssen postpones the distribution of the vaccine in Europe after paralyzing the US administration for six cases of “rare and serious” thrombi.

The health authorities of the United States recommended this Tuesday to stop the vaccination of Janssen after detecting six cases of “rare and serious” clots between almost seven million doses, which represents 0.0008% of all vaccinated people. Following this decision, Johnson & Johnson postponed the delivery of its Covid-19 vaccine in Europe. “The Centers for Disease Control (CDC) and the US Food and Drug Administration (FDA) are reviewing available data on six cases detected in the US among the more than 6.8 million doses administered. As a precaution, the CDC and the FDA have recommended pausing the use of our vaccine, ”says a company statement.

The episode is reminiscent of what happened just a few weeks ago in Europe with the AstraZeneca vaccine, when some countries decided to limit the use of the drug after knowing some cases of cerebral venous sinus thrombosis that could be related to its administration. The European Medicines Agency (EMA) was then forced to take action on the matter. In the case of Jansen’s vaccine, cases of thrombosis have been identified in women between 18 and 48 years old, and symptoms appeared between the sixth and thirteenth day after vaccination, although a causal relationship could not be established.

In any case, in all this tangled history it is striking that the appearance of thrombi is linked to two vaccines, that of Janssen and AstraZeneca, which share the same technology. A fact that does not have to be related to these adverse effects, as pointed out by some experts, but which is still striking for ordinary mortals. In what ways are these drugs similar? How are they different? What relationship are the thrombi that have appeared in some subjects after their administration?

Let’s go by parts. While Pfizer and Moderna are two drugs that use messenger RNA, Janssen and AstraZeneca are based on a modified adenovirus with which they teach the immune system to protect itself from the coronavirus in the event of infection. In the case of the first, a modified and inactivated version of adenovirus 26 is used, which causes the common cold in humans. In the second, a chimpanzee cold adenovirus is used. In addition, both vaccines are stored at a temperature between two and eight degrees.

But up there. The Janssen vaccine was (and is) the great hope to give the definitive acceleration to the vaccination plan in Spain because, unlike the AstraZeneca, it uses only one dose. In other words, the 300,000 doses that Spain expected to receive this week would be equivalent to about 600,000 of the Oxford vaccine. Why does this vaccine use only one dose and the rest two? What happens in our body with two-dose vaccines is that with the first prick a response is activated that, being primary, is less powerful, as Africa González, former president of the Spanish Immunology Society, explained to EL ESPAÑOL .

“It activates the effector response and the generation of memory cells begins, but we need the second dose to be able to expand and generate a greater, more effective and longer-lasting response.” But nevertheless, This is not to say that the Janssen vaccine is worse. Clinical trials have shown that this drug is 66% effective in reducing disease 14 days after the puncture. The flu, for example, does not exceed 50%.

In the case of the Oxford drug, this efficacy is around 75% with the first dose (but requires a second booster dose). It should also be noted that the trials conducted with J&J have occurred months after those of Pfizer and Moderna, in a context in which different variants of the coronavirus have proliferated all over the planet and where it has also shown efficacy.

And the thrombi?

In any case, and despite the very rare cases of thrombi related to the administration of both drugs, specialists insist that the benefit is much greater than the risk. “If it is confirmed that these six cases have no alternative explanation and are linked to the Janssen vaccine, we are talking about less than one case per million. That is to say, the benefit-risk ratio of vaccination continues to favor vaccination, even if it is confirmed that this vaccine is linked to this picture of cerebral thrombi associated with thrombocytopenia ”, points out Federico Martinón-Torres, member of the WHO Vaccine Advisory Committee and researcher at the Hospital Clínico Universitario de Santiago.

According to Martinón-Torres through his Twitter account, the “most plausible hypothesis” has to do with a reaction of our immune system similar to that which occurs with heparin, “which active our platelets (cells in the blood) abnormally, giving rise to thrombi and bleeding ”. The specialist refers to the syndrome of “vaccine-induced prothrombotic immune thrombocytopenia” (Vipit), as it has been baptized by a group of researchers led by hematologist Andreas Greinacher, from the University of Greifswald (Germany), who has been related with the AstraZeneca vaccine and whose scientific work was first published in the repository Research Square and a few days ago in the magazine The New England Journal of Medicine.

This syndrome, as reported by EL ESPAÑOL, occurs after “an unusual combination of symptoms” These include generalized blood clots and a low platelet count. “It resembles a rare side effect of heparin, a known anticoagulant heparin-induced thrombocytopenia (HIT),” the authors explain in an article of the magazine Science. The study, in any case, is the best approximation that exists to this rare disorder to date, although its results are still premature to draw solid conclusions and have only to do with the Oxford drug.

As Martinón-Torres points out, “the Janssen and AstraZeneca vaccines have in common the antigen used (protein S) and that the vector they use is a non-replicating adenovirus (adenovirus 26 in J&J) and chimpanzee adenovrius at AstraZeneca ”, writes Martinón-Torres. “The possibility that the vector is generically the cause is not very likely, and the only mechanism that is shuffled in theory is that the vaccine was administered by mistake intravenously instead of intramuscularly “.

According to the specialist at the Hospital Clínico Universitario de Santiago, in the event that it was related to protein S, “This would be common to all vaccines available on the market [cosa que no está ocurriendo]Although there are small variations in the genetic sequence, it also seems difficult to explain it there ”.

The health authorities are studying the adverse effects related to both vaccines, but for the moment they have not been able to establish a causal link or risk factors that predispose to suffering this type of very strange thrombi that have been related to both vaccines. “In any case, we must be cautious, trust the authorities, and not lose perspective: We are talking about potentially very rare vaccination-related effects. All drugs have effects and what is involved is the risk-benefit ratio, which in the case of vaccines is clear, because they prevent 100% of severe forms and death from Covid-19, a disease that causes blood clots in 10-15% of those infected ”.