An Inserm and AP-HP clinical trial launched on May 28 aims to “compare the immunological efficacy of the standard vaccine regimen with two doses of the same mRNA vaccine to a regimen combining two different mRNA vaccines”, according to a press release from the AP-HP and Inserm published Wednesday, June 2. Clearly, can we get vaccinated with Pfizer-BioNtech for the first dose, then with Moderna for the second, or vice versa?
Two modes of action …
While waiting for the answer, let’s come back to the mode of action of each of the Covid-19 vaccines currently used in France.
Unlike traditional vaccines which inject dead (or inactivated), attenuated or cut into pieces viruses, all Covid-19 vaccines use a piece of the virus’s genome (DNA or RNA). But there are two categories:
– The AstraZeneca and Janssen vaccines both contain a live virus. But this is not SARS-CoV-2. “The trick is to use an adenovirus, that is to say another virus that is harmless,” explains Sandrine Sarrazin, researcher at the Inserm immunology center in Marseille. “We remove its chromosome and replace it with a small piece of DNA from SARS-CoV-2 which will allow the production of the spike protein.” This protein is the key that allows the virus to enter our cells. And it is also the target that our immune system must learn to fight. In this vaccine, “the adenovirus serves as a vehicle to transport the small piece of DNA into our body”.
– In the case of mRNA (messenger) vaccines, such as those from Pfizer-BioNtech and Moderna, the same principle is applied. The difference ? “The vehicle is not another virus but a lipid bubble (a droplet of fat) and instead of a piece of DNA, it is an RNA molecule that is injected”, she explains. . That is why, in principle, they should be interchangeable. What the Inserm trial must verify.
… the same idea
In the two groups of vaccines, the idea remains the same: “To enter into our cells the small piece of the genetic code of the virus which allows the manufacture of the spike protein, otherwise called the spike protein”, specifies Sandrine Sarrazin.
Thus our own cells can manufacture this protein thanks to the manufacturing plan that we inject into it via the vaccine.
It is therefore as if we “transfer into our own organism the work that is usually done in the laboratory for traditional vaccines”.
In the event of the appearance of a variant, the laboratories are able to modify the vaccine according to the mutations.
This explains in particular the rapid development of these new vaccines.
Because for traditional vaccines, “you have to cultivate the virus, have the protein you need produced by an insect cell in an incubator,” she explains. So more time is needed. Not to mention that these proteins must also be purified before injecting them into the patient.
Another advantage: “In the event of the appearance of a variant, the laboratories are able to modify the vaccine according to the mutations found in the genetic code of the virus thus modified”, explains Sandrine Sarrazin.
“BioNtech and Moderna have already announced that in 6 weeks they will be able to modify the RNA that is in the vaccine to adapt it to any variant. This is unheard of ! »Concludes the researcher.
What is a variant?
SARS-CoV-2 is a virus from the coronavirus family. The latter have the characteristic of mutating quickly. In other words, their genome changes, evolves.
How does he do it? “During infection, the virus infects our cells where it multiplies to survive. We can say that it makes photocopies of its genome, ”explains Sandrine Sarrazin, researcher at the Inserm immunology center in Marseille. “When making these photocopies, errors occur. It is these errors that form the mutations and create the variants. “
For the virus, these are changes that often give it the advantage of better resisting treatment in particular. “Unlike humans or other animals whose evolution takes millions of years, viruses evolve very quickly, sometimes in a few hours”, describes the researcher.
This is why it is essential to monitor closely the viruses which infect the population, in order to identify very quickly the appearance of a new variant. The other key in the fight against Covid-19 and its variants is to “limit, through vaccination, the possibilities of circulation of the virus among unvaccinated people”. Indeed, “the variants emerge inside unvaccinated bodies”.
Conversely, “when you are fully vaccinated (2 weeks after the second injection, editor’s note), either you do not contract the virus, or with such a low viral load that there is very little risk of transmitting it. And in any case, the virus is not able to multiply enough to generate a new variant, ”she concludes.