This vocabulary is invited into our daily lives.
For a year - and even more in recent months - the terms “mutation” and “variant” have been regularly associated with Covid-19.
If they seem close, almost synonymous, they actually represent two different notions.
To understand them, we have to go back to the basics.
When a virus replicates in a host, it copies its genome, which is made up of 30,000 small building blocks, called nucleotides.
During this action,
“Almost systematically, it introduces errors, with a frequency of around one error per genome copied.
These errors are at the origin of the mutations ”, explains Yves Gaudin, virologist at the Institute for Integrative Cell Biology at Paris-Saclay and Director of Research at CNRS.
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Mutations are therefore constantly occurring.
In general, this is a unique and very local change.
It may also have absolutely no impact.
But it happens that the mutation results in "changes in the proteins of the virus", continues the virologist.
However, it is the proteins that allow the virus to function.
Three types of mutation
Three scenarios then arise.
This change can create "a less efficient machinery and, in this case, it is not selected", begins Yves Gaudin.
So she disappears almost as quickly as she arrived.
The mutation can also be neutral: it presents no particular advantage for the virus or disadvantage.
It is therefore not eliminated.
And, from time to time, "we have a mutation which somewhat favors the contagiousness of the virus, its infectious nature", describes the research director.
In this case, it is selected.
It is only when there is "an accumulation of favorable mutations" - for example, allowing the virus to be transmitted better in the population - that one speaks of a variant.
“So there must be not just one, but several changes” for this term to be used, he continues.
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And when a variant accumulates a very large number of mutations, it becomes "so different from the initial virus - in terms of properties, pathogenesis, transmission, immune response - that we are then talking about distinct strains", adds Yves Gaudin.