After more than two years of the pandemic, we have reached a situation where the number of infections caused by SARS-CoV-2 is higher than ever. Many of us, despite having received the complete vaccination schedule, have recently suffered from covid-19 and/or know people who are currently in quarantine due to the infection.
However, despite the alarming numbers of infections, the fact that the omicron variant apparently causes milder symptoms than its predecessors could be leading to a trivialization of the disease. Everyone seems resigned to the fact that, sooner or later, they will be infected, which makes some even think that the sooner the better.
Is this correct or are we anticipating our predictions about the behavior of the virus?
Viruses are optimized thanks to their great evolutionary potential
For a virus from animals to cause a pandemic in the human population, it needs to overcome a series of barriers.
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In the first place, it must be able to interact with a molecule on the surface of our cells that allows it to enter them, nothing simple. Once inside, the virus encounters an environment that is less favorable for its multiplication than that which existed in the animal it usually infected. Finally, it is necessary that it find ways to go abroad that allow it to be transmitted efficiently between individuals, so that a stable chain of transmission is generated.
To overcome all these barriers, viruses have a very powerful tool, which is their great evolutionary potential. Every time a virus penetrates a new individual, during the course of infection it will produce billions of copies of its genome. And practically every time one of these copies is produced, at least one error is generated, a mutation, using a slightly more technical term. Therefore, each infected person not only contains an immense number of viruses, but most of those viruses are different from each other.
Various forces operate on this heterogeneous set. On the one hand, natural selection will make the viruses that contain some beneficial mutation the most abundant and, therefore, the ones that will be most successful when it comes to being transmitted.
However, an epidemic is a very complex phenomenon that is also subject to chance. One example is that, of all that variety of viruses that exist in an infected organism, only a few will be transmitted during a contagion, which casts some uncertainty on the subsequent evolution. Another fact that influences which variants will be more successful is the behavior of individuals. A very active person with many relationships will be more successful in transmitting their viruses than one who barely moves.
Can we predict the evolution of viruses?
The short answer is no. However, certain regularities can be established. In general, most viruses naturally tend to improve their transmission, something that has not been an exception in the case of SARS-CoV-2. Very soon, at the beginning of the pandemic, a mutation (the D614G change) was imposed on the virus spike, the effect of which seemed to be to increase its multiplication in the upper respiratory tract, thus favoring contagion.
Subsequently, new variants appeared, the so-called alpha, beta, gamma and delta, which contained mutations that, in some cases, favored interaction with the cell receptor and, in others, hindered the action of antibodies generated in previous infections or induced by the vaccines that were already being applied massively in many countries.
However, none of those variants appeared to be less aggressive than the original virus. In fact, the data indicated rather the opposite: an increased risk of hospitalizations and deaths. Something that did not fit with the behavior of most respiratory viruses, which tend to reduce the severity of symptoms over time.
This tendency to attenuation is easily explained if we think that a virus that manages to multiply optimally, without causing serious illness, will be more successful in spreading, since the carrier is more likely to continue with their normal contacts, who can transmit the virus.
And suddenly came omicron
When in some countries the incidence seemed to have stabilized at really low levels, we heard of a new variant with an unusually high number of mutations (more than thirty in the spike alone). This variant has quickly established itself in almost the entire world, also infecting people who have been vaccinated or who have already had the disease.
This fact is worrying since it is showing the high capacity of the virus to generate mutants capable of eluding previous immune responses. However, despite the difficulties in comparing the severity of omicron with the original virus, there are several studies that indicate that this variant does cause milder symptoms.
The virus is multiplying more
It seems that the expected attenuation of the virus could have arrived, but at the cost of such a high number of infections that its consequences are impossible to assess. The virus is multiplying more than at any other time in the pandemic and that means that it is also experiencing a very high number of mutations.
Most may be irrelevant. But it cannot be ruled out that any of them give the virus an advantage in a way that is detrimental to us. Let’s not forget that when we talk about big numbers, the apparently impossible can become probable.
The number of mutants that a virus can generate is immense, but the good news is that not all of them are viable and not all of the properties of the virus can be improved. For example, the measles virus has many restrictions to generate variants that elude the immune response generated in a first infection, simply because the changes required are incompatible with the entry of the virus into the cell. At the other extreme we have the influenza virus, whose capacity for reinfection we all know.
Some recent studies indicate that one of the endemic coronaviruses that affects our species, called 229E, is capable of causing frequent reinfections, due to its ability to generate variants that are not neutralized by the antibodies produced against previous versions of the virus.
How will the evolution of SARS-CoV-2 be? In my opinion, it is too soon to venture anything, but trivializing a disease about which we still do not know much can be hasty. That fear does not paralyze us, but that it does not lead us to behaviors whose consequences we cannot yet assess.
Ester Lázaro Lázaro, Scientific Researcher of the Public Research Organizations. Specialized in virus evolution, Astrobiology Center (INTA-CSIC)
This article was originally published on The Conversation. Read the original.
