The origin of the Sars-CoV-2 coronavirus, responsible for the pandemic that has already claimed the lives of almost 6 million people worldwide, remains a mystery to researchers and health authorities around the world.
Now, three new studies, one published last Friday (25) and two last Saturday (26), point again to the Huanan animal market, in Wuhan, as the epicenter of Covid. The three surveys have been made public in online pre-print repositories and are still awaiting peer review.
The hypothesis that the place, which sold live animals along with fresh and frozen animal products, was the initial focus of the pandemic had already been raised several times, but the final proof, or the so-called “patient zero”, remains unknown.
The extensive report published by the World Health Organization (WHO) in February 2021, presenting four different scenarios for the transmission of the virus to humans, pointed to the natural origin from an animal host as probable to very probable, but the team of research had ruled out the animal market as the epicenter.
However, as a survey by evolutionary biologist Michael Worobey pointed out, the first case officially recognized by the WHO was not the first in fact. From an analysis published in the journal Science in November 2021, Worobey showed that, in fact, the first person infected was a fish seller at the Huanan market.
From there, Worobey and colleagues investigated the distance of the 174 cases recognized by the WHO at the beginning of the pandemic, in December 2019, from the market. Using latitude and longitude data available for 156 of them, the scientists found that the majority of cases occurred near or in the region surrounding the market, including cases classified as not directly linked to the location by the health entity.
The researchers also collected geolocation data from the social platform Weibo, used in January and February 2020 by 737 people with symptoms of Covid. The heat map (density of cases) also showed a greater concentration in the region close to the market.
To assess whether this proximity could not be a coincidence, the scientists calculated the average distance of the population of Wuhan to the market in Huanan, as well as the distance between the first confirmed WHO cases linked to the market, the unlinked cases, and the cases coming from the Weibo app. The average distance from the population to the market is 16.9 km, while the distance from Weibo cases found was 8.26 km.
Regarding the cases recorded in the WHO report, the average distance of all cases (n=174) was 3.95 km, while that of cases directly linked to the market was 5.33 km, and unlinked cases 3 .75 km.
According to the researchers, these findings would indicate that it is “extremely unlikely” that the cases observed in December were so close to the market for reasons of population density. Also, the Weibo platform cases, from January and February 2020, also had a greater proximity to the place compared to the population as a whole.
Thus, they conclude that the statistical analysis confirms the hypothesis that “the first Covid cases were highly concentrated in, or close to, the Huanan market”.
On Friday (25), researchers from the Center for Disease Control and Prevention (CDC) in Wuhan released the results of RT-PCR analysis to detect the coronavirus in 1,380 samples collected at the Huanan market in January 2020.
Shortly after the first cases of “pneumonia of unknown cause”, which would later become Covid-19, in December 2019, the Chinese authorities closed the Huanan animal market and disinfected the place, which remains closed until the end of May. time. Scientists collected samples from 923 surfaces, such as countertops, walls, floors and even manholes, and 457 animal samples, including carcasses, strays and animal feces at the site.
Although samples from animals that were sold on the market tested negative for Sars-CoV-2 – the main reason why the intermediate host of the virus is still unknown before jumping to humans – the Chinese found traces of Sars-CoV -2 at several points in the market, mainly in the west wing, which is on the left side of the Xinhuan Highway, which cuts the market in two.
In Worobey’s study, the authors also pointed out that some of the markets that offered live animals included species such as raccoons, badgers (genus Arctonyx) and foxes, which are possible coronavirus reservoirs.
As in the research by Worobey and colleagues, Chinese people saw a higher incidence of traces of the coronavirus in stalls selling live animals, most concentrated in the southwestern part of the market.
Despite the WHO’s efforts to test more than 80,000 individuals of different animal species, the authors reinforce that those most susceptible to the coronavirus, identified by them as being sold in the Huanan market in November and December 2019, were not included in the sample. .
Early strains of Sars-CoV-2
In addition to evidence of a high density of cases in the vicinity of the market and the traces of Sars-CoV-2 found at the site, the authors identified the two initial strains of the coronavirus, known as A and B, also linked to the Huanan animal market in beginning of the pandemic.
The third study published last Saturday (26) indicated two distinct jump events from these strains to humans. The first, involving lineage B, which became predominant during the Covid pandemic, took place in late November and early December 2019, while lineage A infection occurred a few weeks later.
In the research by Worobey and colleagues, the authors found evidence of cases occurring with lineage A, which had not been previously associated with the Huanan market, within a distance of less than 1 km from the site.
Similar results were found by Chinese researchers at the CDC, who found the presence of strain A in samples collected in the market environment.
Thus, the data again corroborate the circulation of the two strains at the beginning of the pandemic at the market place. Whether one strain was derived from the other — the difference between the two is just two mutations — or whether strain A was brought to the site by an infected person remains to be seen, scientists say.
Nor is it possible to determine which — or which — animal species are directly linked to the leap from the coronavirus to humans, but the new findings increasingly point to an origin of Sars-CoV-2 in the Huanan market.