Wildlife Markets: Origin of Covid-19 Finally Confirmed?

A few weeks after the media wave was sparked by the American economist Jeffrey Sachs, whose comments tend to support the thesis of a biological leak from a research laboratory, two studies published the same day in the journal. Science seems to rule out this hypothesis.

After the preliminary version was published last February, these two studies have meanwhile benefited from peer validation, i.e. the respective manuscripts have been reviewed by scientists in the related field. critical analysis, including assumptions, methods, data, and results.

First cases of Covid-19, centered around Huanan market

The first study (Worobey et al.), led by viral evolutionary scientists Michael Worobey and Kristian Andersen, professors in the Department of Immunology and Microbiology at the Scripps Research Institute in San Diego, California, was the kind of epidemic epidemiology.

The authors tracked the geographical circulation of the virus, including the location of the first 174 cases identified by the World Health Organization (WHO) in December 2019 – 155 of them in the city. Wuhan city of China – as well as samples collected at different locations.

According to this study, all the first identified cases of Covid-19 were centered around the Huanan market in Wuhan: “In a city of more than 7,700 square kilometers, the area most likely to be home to someone with one of the world’s first COVID-19 cases is an area several blocks wide, with the Huanan market. internal“, Emphasizes the lead author in a press release.

Infected sellers pollute more?

The cases, on the other hand, are then more homogeneously located in the territory of this city of about 11 million inhabitants. As a result, by January and February of the following year, patients are no longer just hanging around markets but especially in more densely populated areas of the city, the researchers note.

Among the first patients affected, a very high percentage of those who have no connection to the market, that is, those who do not work there or do their shopping there, actually live nearby. Thus, according to the authors, the Huanan market vendors were infected first, before infecting members of their community.

It’s important to realize that all of these cases were people who were identified because they were hospitalized.“, Worobey Point.”None of them is a mild case, which can be identified by knocking on the doors of people living near the market and asking them if they are sick. In other words, these patients are counted because they are in the hospital, not because of where they live. (may have falsified research, editor’s note). “

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In addition, the study’s authors adopted a statistical method that included phasing out of their analysis part of the cases, to see if the results were modified. Even excluding three-quarters of the patients, the authors note that the remaining cases lived closer to the market than what could be observed by chance.

Finally, the researchers are also interested in samples collected by Chinese researchers on the floors and surfaces of the Huanan market after it closed in early 2020.

However, by rebuilding the market plan, they realized that the samples that tested positive for SARS-Cov2 were more suitable for the Western area, where there are stalls where live wildlife is sold. Includes fox, Asian badger and raccoon dog (‘raccoon dog‘ in English).

A previous study (Xiao et al., 2021) identified about 47,000 animals – of 38 different species – sold on the Huanan market in the months leading up to the Covid-19 pandemic.

Two events of animal-to-human transmission

The second study (Pekar et al.), led by Jonathan Pekar and Joel Wertheim of the University of California at San Diego and Marc Suchard of the University of California at Los Angeles, was based on analysis of viral genomes collected from diseases first multiplier. hospitalized for this disease in the early weeks of the outbreak.

Viral DNA sequences carry the traces of their evolution, because their genomes accumulate mutations over time, at a certain rate. Thus, the “molecular clock” approach involves estimating the time that separates the emergence of two viral strains (or two species, when it comes to bacteria, plants, and animals) from their common ancestor. they.

However, at the beginning of the pandemic, two strains of SARS-CoV-2 – very distinct from each other – were identified.

By analyzing their genomes, the researchers concluded that if the mutation rates of the two virus strains were the same – which they consider the most likely hypothesis – then the two events of zoonotic transmission The man-to-man must have happened in the same time period. , that is, from late November to early December 2019.

The authors even suggest that other zoonotic infections may have occurred, but were not pathogenic, and therefore would have gone unnoticed.

The controversy can continue

However, these two studies do not solve the mystery of what may have happened before market wildlife was initially infected with the virus to market vendors: whether a bat transmits the virus. virus to an intermediate host, such as a wild fox or raccoon dog? Does the virus circulate in a farm’s herd?

The authors therefore recommend studying wildlife capture and trafficking networks in Asia, but also conducting preventive screening of animals sold live in markets to avoid future pandemics. . To date, more than six million people have died from Covid-19.

It is therefore uncertain whether the controversy can be resolved – this requires determining whether the “patient”, i.e. the first human infected, or another mediator, specifically were the animals that were initially infected by bats.

However, one of the authors, Professor David Robertson, a virologist at the University of Glasgow, told BBC News he hopes that their work”will correct the false claim that the virus came from a laboratory“.

We are now as certain as possible, based on the fragmentary evidence we have, that this is a transmission event that has occurred in the market.“, many Professor Stuart Neil, Kings College London, who did not contribute to these studies – but who has previously collaborated with several authors for a previous publication (Holmes et al. 2021) .

For this researcher, the lab leak hypothesis serves as an explanation for the emergence of the pandemic.”put us at risk of letting it happen again because we focused on the wrong problem“.

The risks of transmission (viruses) from animals to humans, a phenomenon known as ‘contagion’, whether during hunting, handling of animals or consuming wild meat are real and likely. destructive power.“, warned a few weeks ago Véronique Chevalier and François Roger, researchers at CIRAD (Conversation).

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