Washington (AFP) – An animal market in Wuhan, China was really the epicenter of the Covid pandemic, according to two new studies published in the journal Science on Tuesday, which they claim have tipped the balance in the debate over the origin of the virus.
The answer to the question of whether the disease was transmitted naturally from animals to humans or was the result of a laboratory accident is considered vital to preventing the next pandemic and saving millions of lives. The first paper analyzed the geographic pattern of Covid cases in the first month of the outbreak, in December 2019, showing that the first cases were closely clustered around the Huanan market. The second looked at genomic data from the first cases to study the early evolution of the virus, concluding that it was unlikely that the coronavirus was widely circulating in humans before November 2019. Both were previously posted as “preprints” but have now been peer-reviewed and published in a peer-reviewed journal. Michael Worobey of the University of Arizona, who co-authored both papers, had previously called on the scientific community in a letter to be more open to the idea that the virus was the result of a lab leak. But the findings have led him “to the point where I also now believe it is not plausible that this virus was introduced by any means other than the wildlife trade in the Wuhan market,” he told reporters on a call about the findings. Although previous research had focused on the live animal market, researchers wanted more evidence to determine that it was truly the progenitor of the outbreak, as opposed to an amplifier. This required a neighborhood-level study within Wuhan to be more certain that the virus was “zoonogenic” — that it jumped from animals to humans. The team in the first study used mapping tools to determine the location of the first 174 cases identified by the World Health Organization, finding that 155 of them were in Wuhan. Moreover, these cases were closely clustered around the market — and some early patients with no recent history of visiting the market lived very close to it. Mammals now known to be infected with the virus — including red foxes, badgers, pigs and raccoon dogs — were all being sold live in the market, the team showed.
Two introductions to man
They also linked positive samples from patients in early 2020 to the western part of the market, which sold live or freshly slaughtered animals in late 2019. The tightly contained early cases contrasted with how it radiated across the rest of the city in January and February, which researchers confirmed by examining social media check-in data from the Weibo app. “This tells us that the virus was not circulating cryptically,” Worobey said in a statement. “It really came from that market and spread from there.” The second study focused on resolving an apparent discrepancy in the early evolution of the virus. Two lineages, A and B, marked the early pandemic. But while A was closer to the virus found in bats, suggesting that the human coronavirus came from that source and that A caused B, B was found to be much more present in the market. The researchers used a technique called “molecular clock analysis,” which relies on the rate at which genetic mutations occur over time to reconstruct a timeline of evolution — and found it unlikely that A caused B. “Otherwise, the A lineage would have to evolve in slow motion compared to the B lineage virus, which just doesn’t make biological sense,” Worobey said. Instead, the likely scenario saw both jump from market animals to humans on separate occasions in November and December 2019. The researchers concluded that human circulation was unlikely to have occurred prior to November 2019. Under this scenario, there were probably other animal-to-human transmissions in the market that failed to manifest as Covid cases. “Have we disproved the lab leak theory? No, we haven’t. Will we ever know? No,” said co-author Kristian Anderson of the Scripps Research Institute. “But I think what’s really important here is that there are possible scenarios and they’re plausible scenarios and it’s really important to understand that possible doesn’t mean equally likely.” © 2022 AFP
