Paris (AFP)
Viruses don't think, but they can adapt to their environment in order to survive.
A capacity that could explain the worrying emergence thousands of kilometers away of several more contagious variants of the Covid-19 virus.
Like all viruses, SARS-CoV-2 mutates: when it replicates, errors occur.
Most of these mutations are harmless but some can give him an advantage for his survival.
This is the case of the three more contagious variants recently identified in the United Kingdom, South Africa and Brazil, which appeared in the fall, a few months apart, when no significant mutation had been detected during the first months of the epidemic.
A coincidence?
It is indeed partly a matter of chance, according to experts, but not only.
"When we reduce the number of contaminations, we restrict the playing field of the virus" and therefore the probabilities of a problematic mutation, explains Dr Emma Hodcroft, epidemiologist at the University of Bern.
On the contrary, in a context of high circulation of the virus, "we maximize the opportunities for the virus to meet a certain scenario or a certain person who by chance could lead to something that we did not want", she continues, evoking a game of "roulette".
"It is a combination between on the one hand the quantity of virus which circulates, the number of times we throw the dice, and on the other hand the environment of the virus", specifies Wendy Barclay, virologist at Imperial College of London, referring by environment, to a world where the virus has spread widely.
"This is the time when we can expect the appearance of variants affected by the immune response because the level of immunity to the virus around the world increases, through infections and vaccination," he explains. her at a press conference.
- Immunocompromised patient -
"In two of the places where the problematic variants emerged, South Africa and Brazil, there was already a significant level of immune response in people who were already infected and recovered," she notes.
But some scientists doubt a link between high seroprevalence and the appearance of variants.
"It is more likely that the evolution of the virus took place inside a patient," said Bjorn Meyer, virologist at the Institut Pasteur in Paris.
In particular a patient with an immune deficiency, as some researchers suspect for the English variant.
"When a patient is immunocompromised, the virus can stay on longer," Bjorn Meyer told AFP.
While the Covid-19 virus survives an average of ten days in an individual, studies show that some patients have harbored a live virus several weeks or even months after infection.
But even immunocompromised, a patient defends himself a little against the virus, without succeeding in eliminating it completely.
Faced with this "immune pressure", "the virus is forced to mutate. Either it mutates and learns to escape this partial immune response, or it dies," continues Bjorn Meyer.
So why didn't a more transmissible variant emerge earlier, during the first months of the epidemic?
"This is where chance comes in," notes the virologist.
"There are not that many immunocompromised people. And at the start of the epidemic, there were fewer cases and people who knew they were immunocompromised were particularly protected, isolated."
- May the best win" -
The situation could be different in areas where there are more people who are immunocompromised, or do not know it.
"The emergence of a variant of SARS-CoV-2 in August in South Africa, one of the countries in the world most affected by HIV infection, could be the result of more intense and prolonged viral replication in the body of people living with HIV, promoting the accumulation of mutations, "notes the Academy of Medicine in France.
A "valid" hypothesis, replies Bjorn Meyer, even if it is difficult to identify precisely the origin of a variant.
In any case, whatever the precise conditions for the emergence of variants, natural selection comes into play.
"It is a normal process of competition in which the best wins, the one who is the strongest, the one who can be transmitted the best, is the most able to perpetuate the viral species in question. It is a typically Darwinian process. which guides the evolution of the living world, ”summarized Belgian infectious disease specialist Yves Van Laethem at a press conference.
This is why the researchers believe it is likely that other problematic variants will emerge.
They may even be there already.
"Because the total number of cases continues to increase exponentially, it is not difficult to argue that more problematic variants have emerged during the winter without being detected than variants that emerged in the fall and who are now on our radars, "said Carl Bergstrom, professor of biology at the University of Washington, on Twitter, pleading for better surveillance.
© 2021 AFP