Focusing our attention on the Sars-Cov-2 virus with which we have lived for two years, we will review the emergence and development of new viral strains in the light of evolution.

RNA viruses can be highly variable

Evolution has two engines that drive it: genetic variation and natural selection. Genetic variation is caused by errors in the replication of genetic material. Given the amount of genetic material copied in each generation and the speed of the process, these errors are surprisingly rare. However, the enormous number of constantly multiplying microorganisms or viruses makes this condition rare. Thus, mutations accumulate in the population.

Replication of genetic material is more precise in organisms and structures that use deoxyribonucleic acid (DNA) as information support because the machinery that performs it (DNA polymerases) may have error correction mechanisms.

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However, when the genetic material is ribonucleic acid (RNA), the replication mechanism is less precise and errors are more frequent. As a result, RNA viruses are more prone to mutation than DNA viruses or bacteria and other cellular organisms.

There are exceptionally mutable RNA viruses that form different population clusters within the same infected individual. This is what happens, for example, with the hepatitis C virus. However, the concept of “half-species” is applied to explain the coexistence of multiple non-replaceable variants of the virus.

The coronavirus has the ability to adapt to different environments.

Natural selection, on the other hand, is the process by which some organisms have greater reproductive success than others in a given environment and can become dominant or privileged to displace competitors.

As it is easy to understand, those mutations that provide greater reproductive success in an environment will tend to become more frequent in the population, and the organisms that have them will be better adapted to that environment. Indeed, the most reproductively successful variants prevail, and if a variant can colonize a new environment, it will do so and dominate it, at least temporarily.

Coronaviruses are small RNA viruses with high variability. This allows them, as a group, to adapt to different environments (infect different animal species) and continually produce new variants whose relative abundance will depend on their reproductive success.

In this way, since the SARS-CoV-2 coronavirus first infected people in Wuhan, where the pandemic originated, new variants of the virus have been constantly emerging. Like waves in the sea, its prevalence has been steady rising and falling.

These variants are cataloged in lineages, forming detailed genealogies that allow for rigorous epidemiological monitoring of the pandemic, just as it is done for others such as influenza or AIDS.

As of today, it is estimated that there are close to 500 million coronavirus cases in the world, with more than six million deaths. Given the millions of viral particles produced in each infected person and the frequency of mutations, the emergence of new variants of the virus is an inevitable phenomenon that will remain constant in the future.

Arrival of a new variant series

Recently, there has been talk of a new line of Sars-Cov-2 variants with names starting with the letter X: XA to XS. These variants are recombinants of other strains described above.

Its origins are not as I have described in the previous paragraphs, but were created during the coinfection of the same cell by two coronaviruses from different strains. During the replication of two viruses in the same cell, their genetic material may combine and a new virus may arise that combines the characteristics of both parents. If this new virus is more reproductively successful than others in the population, it will predominate and become widespread.

The new recombinant strains of public interest are designated XD, XE and XF and have been predominantly identified in France, Denmark and the United Kingdom.

All three contain material from the BA.1 strain (ómicron) and, in the case of the XD and XF variants, the delta variant. Since these variants are detected with a certain abundance in the aforementioned countries, their prevalence is expected to increase as they have sufficient reproductive success to stand out.

Current data suggest these variants may be more contagious than their predecessors, but there is no significant data suggesting that they will be more lethal than we know.

Recombination of viral genomes is a process that requires co-infection and formation of a new and successful recombinant molecule. The process takes place because of the high number of viruses and infections making recurrences infrequent. The formation of new viruses by this system is the source of recurrent strains of pandemic influenza between the ages of 20 and 30 years, which arise as a result of the mixing of influenza viruses of different origins.

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In the case of the influenza virus, the process is favored by splitting its genome (also RNA) into several molecules. In the case of the coronavirus, the process should be less frequent, as its genome is organized into a single molecule and recombination is necessary, not just packaging errors that lead to genome entanglement in the influenza virus.

And here is the current situation: new variants are emerging that fall into the epidemic monitoring system of the European (ECDC) and North American (CDC) infectious disease control centers.

The ECDC differentiates three levels to organize the intensity of cascade monitoring of new viral variants: variants under monitoring (XD and omicron BA.3), variants of interest (ómicron BA.4 and BA.5 strains) and related variants (delta) variant and BA. .1 and BA.2 omicron strains). The XE and XF variants have not yet reached relevance to be classified as variants under surveillance.

The World Health Organization also includes only the XD variant among those classified as surveillance.

We cannot know exactly what the future impact of these new variants will be on population and epidemic evolution because evolution is a combination of chance and necessity; however, we can have some expectations based on what we have learned from evolution and past epidemics.

What to expect from this variant

We can define the virulence of a pathogen as the number of severe or very severe cases relative to the total number of infected people. With the necessary precautions, the virulence of pathogens in general tends to decrease with the duration of coexistence with the host.

This can be explained by the reproductive success of a pathogen, the less harm it does to the host and therefore the more it can transmit the pathogen. A dead dog does not transmit rabies. Therefore, less virulent variants tend to spread better than others that limit or eliminate the mobility of the sick animal or person, and tend to dominate the community over time.

On the other hand, the immune system learns to deal with this virus by controlling its replication and modulating the response (in the case of coronavirus infection, it is essential in exacerbating the disease).

Therefore, and as a result, we should expect new coronavirus variants to continue to emerge, the prevalence of which jumps to the press and private scrutiny of international organisations. But it can be reasonably expected that these new variants tend to be less lethal. In this way, the successive waves of the pandemic should be softer and the hospital response, if any, will not be significantly affected.