Science

Vaccines and blood clots – 13/04/2021 – Esper Kallás

The discussion about vaccines for Covid-19 has recently been marked by the appearance of some cases of thrombosis, which is characterized by the formation of clots that can obstruct the circulation in some people’s blood vessels.

Covid-19 prevention vaccines have been introduced in several countries that employ different strategies to simulate the encounter of the new coronavirus with the immune system.

One of these strategies manipulates a harmless virus to take part of another into the body. These are called “viral vectors” which serve as the “Trojan horse”. In the case of the vaccine against Covid-19, the viral vector artificially carries the outermost part of the new coronavirus, its “spike”, in its genetic material.

To further ensure that the viral vector is harmless, modifications are made to prevent it from replicating. Thus, the vector can go through a single round of protein production in human cells, sufficient to stimulate the immune system to produce antibodies to neutralize the new coronavirus.

Two vaccines already used against Covid-19 use this strategy: Covishield, made by AstraZeneca, and the vaccine by Janssen, a Johnson & Johnson company. The first uses an adenovirus identified in chimpanzees while the second uses adenovirus 26. To the surprise of scientists, both may be associated with rare cases of thrombosis. There is no information yet about a third vaccine, Sputnik V, which also uses adenoviruses as vectors.

Several countries have restricted the use of Covishield following the announcement of cases of thrombosis in people vaccinated with it. In the United States, the Federal Department of Health temporarily suspended the use of the Janssen vaccine after six similar cases. The characteristics of the episodes are the same: thrombosis, most common in young women, between 5 and 30 days after vaccination.

Some studies suggest that the adenoviruses used may have stimulated the production of an antibody that can attack the PF4 protein found in platelets, fragments of cells that work in clot formation. If this relationship is confirmed, it is judged to be quite unusual: 4 cases per million vaccinated, based on the numbers available so far. It would therefore be a very rare adverse event.

Given the strength of Covid-19, which can also lead to thrombosis, there is no justification in abandoning the use of these two vaccines that use viral vectors, at least until a sufficient number of doses of other vaccines that do not use adenovirus are available .

Rare adverse events are not uncommon with vaccinations. Immunization from yellow fever can cause disease in 2.5 people per million vaccines, while Sabin, an oral polio vaccine, can cause one case of muscle weakness for every 3.5 to 4 million children given the vaccine. Still, both of them remain busy as their overall positive effects are enormous.

While there are concerns, identifying these specific cases of thrombosis and their possible association with vaccines against the new coronavirus was only possible thanks to the diligence of research projects and pharmacovigilance.

The care taken in the use of vaccines against Covid-19 has proven to be exemplary, as the suspicion could be recognized immediately. As with all public health decisions, a rigorous cost-benefit analysis must be applied to the use of vaccines, taking into account the weight of these potential side effects.

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