Where did the new coronavirus come from: SARS-CoV-2

While some scientists are struggling to develop a vaccine that can stop the rapid progress of the COVID-19 pandemic, others are trying to unravel one of the main mysteries of modern virology: where did SARS-CoV-2 come from, which gave rise to a pandemic. At the same time, both the former and the latter admit that it is difficult to find an effective way to combat a disease if little is known about the nature of its pathogen.

The World Health Organization has formed an international research team of scientists to work together to find an answer to the question of the origin of the new coronavirus. Nevertheless, a variety of theories are already being considered in scientific circles: from the natural mutation of the virus to the leakage of SARS-CoV-2 from the laboratory.

Theory 1: Bats are to blame

SARS-CoV-2 is the third coronavirus that has caused an epidemic in the last 20 years. The first was SARS-CoV in 2002, the second was MERS-CoV, which gave rise to a mass incidence in 2012. But the causative agent of COVID-19, despite its common features with previously known coronaviruses, raises many questions among specialists, to which there are no unambiguous answers yet.

When the outbreak in Wuhan became known, bats were immediately blamed for COVID-19. The fact is that this family of coronaviruses mainly circulates among these animals. Nearly 500 species of bats are known to carry coronaviruses. In this case, the same animal can be infected simultaneously with several strains of the virus, which only contributes to the recombination and evolution of the virus. As a result of such changes, strains acquire the ability to overcome species barriers.

As for SARS-CoV-2, the genome of this virus is 96% identical to the virus found in bat guano in 2013 (RaTG13). But with its “predecessor” SARS-CoV-1, the latest coronavirus is only 79% similar, and with MERS-CoV even less – by 50%. That is, the new coronavirus genetically has much more in common with the strain that previously circulated only among bats, and much less with strains that previously affected humans. In addition, the fact that in 99.98% of cases the samples of the new coronavirus in infected patients are identical, serves as a basis for scientists to assert that SARS-CoV-2 has recently been circulating in the human population. Chinese scientists have compared 5 genomes of SARS-CoV-2 and 276 genomes of coronaviruses that previously affect humans and animals. The researchers found that new viruses emerged from a common ancestor around 2018 due to recombination of bat coronavirus and likely Chinese snakes. However, a group of scientists have challenged the theory of their Chinese colleagues that snakes can become infected and carry a new virus.

It would seem that everything is simple in the version with bats, but there is a missing link in this story, because of which many questions arise. Previously, no case of direct infection from bats to humans was known. In the course of a viral mutation, there should have been a so-called intermediate host, in whose body the virus mutated to a form capable of infecting humans. At least, it was in this scenario that the mutation of the two previous coronaviruses occurred.

No confirmed animal mediators have been identified for the COVID-19 pathogen. According to one version, pangolins could be the link. But this hypothesis also raises more questions than it answers.

Simply put, between the RaTG13 virus diagnosed in bats and SARS-CoV-2, there must be another one - an intermediary. But so far this has not been found. And until an intermediary is found, the theory that the COVID-19 pandemic is the result of a virus leak from the laboratory also has a right to exist.

Theory 2. SARS-CoV-2 is a “child” of the laboratory

First of all, this idea is suggested by the fact that near the Wuhan market, where the first cases were diseases, there is a laboratory specializing in the study of coronaviruses. In particular, its employees studied the possibilities of overcoming species barriers by viruses.

Supporters of this theory put forward several options for the possible development of events. According to one hypothesis, a virus adapted to other animal species could have leaked, and later it mutated to a strain dangerous to humans. Or SARS-CoV-2 may initially have a synthetic nature. This assumption was put forward by the intelligence services of many countries, including Germany, Great Britain, Canada, and the USA. Moreover, humanity has already encountered cases of leakage of viruses from laboratories. This happened to the Marburg virus, the causative agent of severe hemorrhagic fever, in 1977.

What are Wuhan virologists guilty of?

The main argument in favor of the theory of the Chinese origin of SARS-CoV-2 is the fact that over the past 15 years, Wuhan University has been researching bat coronaviruses. About 4 years before the start of the pandemic, Dr. Shi Zhengli, who leads the study of coronaviruses, together with American researchers, came to the conclusion that viruses that genetically resemble SARS can be directly transmitted from bats to humans, causing a disease that is not amenable to therapy. And in March 2019, the scientist foresaw an epidemic of SARS, which could begin in China.

Interestingly, the laboratory where coronaviruses were studied is located not far from the Wuhan market, which became the initial link of the future epidemic. At the same time, the laboratory categorically denies allegations of developing a strain and leaking the virus. At the same time, according to one theory, patient zero was not a market visitor, but Huang Yanling, an employee of the Institute of Virology in Wuhan. But soon China "cleaned up" all references to this patient.

What do Americans have to do with the new coronavirus?

Back in 2013, researchers discovered a SARS-like virus, SHC014-CoV, targeting horseshoe bats. But 2 years later, virologists from Wuhan, together with researchers from the University of North Carolina from SARS and SHC014, created a chimera virus. He could infect not only bats, but also people. In studies on experimental mice, it was found that the virus causes pneumonia. At the same time, there were no drugs against the disease at that time. It was only in 2018 that a vaccine against this virus was created. Soon, the scientific community expressed its dissatisfaction with such experiments, fearing that if the virus leaked out of the laboratory, it would pose a serious danger to humanity.

When the pandemic broke out, many remembered SHC014. However, a group of scientists from North Carolina, Ohio and Pennsylvania denied the connection between SARS-CoV-2 and the chimera virus, arguing that there are significant differences in the genetic codes of these two viruses.

What did French scientists find unusual about SARS-CoV-2?

But the French virologist from the University of Aix-Marseille Etienne Decroly drew attention to the unusual structure of the SARS-CoV-2 genome. French scientists have found that the gene encoding the spike protein contains 4 segments that are not found in other coronaviruses that are genetically similar to humans. It is these segments that are thought to give SARS-CoV-2 its exceptional qualities. At the same time, the first three segments were also found in previously known strains of coronavirus, which, according to scientists, confirms their long-standing origin and evolution. But the last one is unique, previously unknown to scientists. It is possible that the appearance of this fourth segment is the result of laboratory experiments. Scientists believe that it was he who played a key role in the spread of the pathogen among humans.

But at the same time, many experts categorically oppose the theory of the laboratory origin of the virus. In particular, microbiologist Christian Andersen, together with colleagues from Columbia, Sydney and Tulay Universities, studied the genome of the new coronavirus and concluded that it is of natural origin. According to scientists, a synthetic virus would be based on an already known human pathogen, and the molecular basis of SARS-CoV-2 has more in common with the bat virus. What destroys the theory of "biological weapons".

Theory 3. Slow mutation

According to many virologists, SARS-CoV-2 could have appeared according to another scenario. It is assumed that this virus could have originated long before the massive outbreak of infection in 2019.

A strain adapted to the human body could circulate between people for several years. But since the infection was not massive, it did not cause much attention. But then the virus could go through the next stage of mutation, due to which infection occurs much easier, which caused the pandemic. To confirm this hypothesis, it is necessary to study tissue samples from people who died from SARS in the region of the COVID-19 outbreak even before the start of the pandemic.

In addition, some researchers suggest that at first, mutated SARS-CoV-2 may not have caused symptoms of the disease in infected people at all, so nothing was known about it. And only after the next stage of evolution, which could very well have occurred already in the human body, did the virus become truly dangerous for people.

Which of these theories is the most accurate, scientists have yet to determine. In the meantime, research on SARS-CoV-2 continues.

Sources
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