In zombie movies, a “mutant” is scary. But in reality, a virus that “mutates” is normal and neither good nor bad in itself. To date, mutations in Sars-CoV-2 do not appear to have significant consequences.
Why does a virus mutate?
When it enters a cell, a virus replicates: it copies itself in order to spread.
With each replication, errors occur in the copy of the virus genome, such as a computer “bug”. But this error may or may not have a greater or lesser impact on the way the virus behaves.
The mutation can be “favorable” to the virus: it helps it survive better, or “unfavorable” (it weakens it, for example). This is called natural selection.
RNA viruses (genetic material similar to DNA), such as Sars-CoV-2, mutate faster than DNA viruses because their encoding errors are more frequent.
However, coronaviruses mutate less quickly than other RNA viruses: so far, for example, Sars-Cov-2 mutates half as fast as influenza and four times as fast as HIV, according to Emma Hodcroft, epidemiologist from the University of Basel (Switzerland), recently cited in the journal Nature.
Scientists even consider the new coronavirus to be genetically relatively stable. But what matters is whether these mutations have any noticeable effects. What if they make the virus more “dangerous”?
Do these mutations make it more contagious? More pathogenic, that is, more likely to make you sick? More virulent, that is, capable of making people more seriously ill? Less vulnerable to a vaccine? More resistant to our immune defenses?
What do we know about the effects of mutations in Sars-CoV-2?
Genetic mutations in the coronavirus are tracked around the world by researchers, who sequence the genome of viruses they find and share them on an international database, GISAID, a treasure trove of tens of thousands of sequences.
As of yet, there is no clear indication that the virus has mutated in such a way as to significantly alter its effects on humans.
One thing is certain: the new coronavirus is no exception to the rule and “mutates all the time,” Marie-Paule Kieny, virologist and director of research at Inserm, told senators this week.
The “problem” is whether these “mutations change virulence or not? », Added his colleague, Dominique Costagliola, epidemiologist, also heard in the Senate. For now, “we have no data” in this regard.
“So far, that doesn’t mean it never will, it doesn’t appear that these mutations have an influence on the pathogenicity of this virus,” Kieny added.
In data | Our interactive content on COVID-19
In a July study published in the journal Cell (after a preliminary version in the spring), scientists claimed that a mutation made it possible for the most common strain of the virus to infect cells more easily through a change in the protein. S, the “tip” which allows him to enter the cell. According to their hypothesis, this mutation could make the virus more contagious, which would explain its exponential spread.
But many scientists noted limitations, noting that the greater infectivity had only been seen in the laboratory and that this work did not provide evidence that a greater ability to infect cells made it more contagious.
The strictest conclusion is therefore that while this strain is arguably more “infectious”, it is not necessarily more “transmissible” between humans.
The authors also observed, in hospitalized patients, that this strain did not cause a more severe form of the disease.
As for the hypothesis, formulated in particular in August by a scientist from Singapore, that the virus would become less virulent, it has not been scientifically supported. Here again, a lower severity of symptoms can be explained by other factors: fewer comorbidities, better care, etc.
But if he mutates, how can he expect a vaccine?
When its mutations substantially alter the “antigenicity” of a virus, that is, its ability to induce the production of antibodies, vaccines can become less effective.
But as we have seen, Sars-Cov-2 mutates relatively slowly so far, which in this regard could in any event be good news on the vaccine side.
For now, “it doesn’t seem […] that these mutations have a difference in what is called their antigenicity “, according to Marie-Paule Kieny.
“Most of the vaccines were made with viruses that correspond to the first Wuhan strains, but many researchers or industries have tried to test whether the antibodies they manage to generate with their vaccine candidate also neutralize the new viruses. “, She continued,” and they see that indeed, the new viruses are neutralized as well as the old ones.
“So for the moment, it does not seem that these mutations, very real, lead us to say that we will have to do a different vaccine, like the flu, each year,” the scientist also hopes.