The emergence of new SARS-CoV-2 variants has prompted questions in the press such as “Why do viruses mutate?”
The emergence of new SARS-CoV-2 variants as the British, the South African or the Indian in recent months has prompted questions in the press such as “Why do viruses mutate?”. Often, the answer is that “viruses mutate to survive”. However, the correct way to say so would be that, by natural selection, “the strongest mutations survive”, as Quique Bassat, ICREA (Catalan Institution for Research and Advanced Studies) researcher and director of the Malaria Programme, has explained to Verificat. We explain it to you.
Replication is the process by which viruses make copies of themselves, but they are not able to do so alone. They need to infect a cell and take profit from its replication machinery to do it. During the copying process, the genetic information of the virus -RNA for the case of coronavirus- can suffer transcription errors. These will be part of the new generation and are called mutations.
"The virus has no conscience, it doesn’t think or decide," reflects Joaquim Segalés, professor at the Faculty of Veterinary Medicine at the Autonomous University of Barcelona (UAB) and researcher at the Animal Health Research Centre (CReSA). The mutations are not caused by any will, but by errors in the replication of viral genetic information. "Viruses mutate randomly," confirms Quique Bassat.
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In fact, these mutations are very common for viruses carrying RNA (compared to the DNA viruses), because they do not usually have any way to detect the errors. SARS-CoV-2 is a special case within RNA viruses since it has an enzyme able to revise the copying errors and correct them. This makes this virus mutate to a lesser extent than other coronaviruses, but even so, every cycle of copies, which can last a few hours, accumulates mistakes, in such a way that each infected person has a diverse viral population during the infection period. ”If we were able to sequence each one of the viruses inside an infected person, we would observe a cloud of mutations'', points out the UAB professor. The virus “does not change so much to have different behaviours within the same individual, but there are slightly different ones”.
In general, “many of these changes do not have any impact”, affirms Segalés. Others, however, can affect either in a positive or negative way their transmission capacity. This is where natural selection comes into play. Only those mutations representing a serious competitive advantage with respect to other variants, such as greater resistance to our immune system or a higher replication rate, for example, will be favoured and eventually imposed. After a while, an advantaged variant “displaces the others”.
As a consequence, mutations are not a strategy of viruses to survive, but the product of mistakes during their copying process that can lead to changes in the way they interact with our cells and our immune system. If changes jeopardise the virus, their replication probabilities will be reduced, these mutations will be transmitted to a lesser extent to the new generations and the variant will end up vanishing. In the case of being favorable for their replication, they will be more efficiently transmitted, the variant will become more abundant and will be able to impose itself as a new strain.