An animal market in China’s Wuhan was truly the epicenter of the Covid pandemic, according to two new studies in the journal Science, published on Tuesday, which claim to have tipped the balance in the debate over the virus’ origins.
Determining whether the disease jumped naturally from animals to humans or was the result of a laboratory accident is considered crucial to averting 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, December 2019, and showed that the first cases were clustered tightly around the Huanan market.
The second examined genomic data from the earliest cases to examine the early evolution of the virus and concluded that the coronavirus was unlikely to have been widespread in humans prior to November 2019.
Both were previously published as “preprints” but have now been peer-reviewed and published in a respected journal.
University of Arizona’s Michael Worobey, co-author of both articles, previously wrote in a letter urging the scientific community to be more open to the idea that the virus was the result of a lab leak.
But the results brought him “to the point where I also now find it implausible that this virus was introduced into the Wuhan market by any means other than the wildlife trade,” he told reporters on a call about the Results.
Although previous research had focused on the live animal market, researchers wanted more evidence to determine that it was really the precursor to the outbreak and not an intensifier.
This required a neighborhood-level study in Wuhan to be more certain the virus was “zoonotic” – that it jumped from animals to humans.
The team on the first study used mapping tools to pinpoint the location of the first 174 cases identified by the World Health Organization and found that 155 of them were in Wuhan.
In addition, these cases clustered closely around the market – and some early patients who had not recently visited the market lived very close to it.
Mammals now known to be infectious with the virus – including red foxes, pig badgers and raccoon dogs – were all sold alive at the market, the team showed.
– Two introductions to people –
They also linked positive samples from patients in early 2020 to the western part of the market, which was selling live or freshly slaughtered animals in late 2019.
The narrow early cases contrasted with their broadcast to the rest of the city through January and February, which the 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 originated in that market and spread from there.”
The second study focused on resolving an apparent discrepancy in the early development of the virus.
Two lineages, A and B, characterized the early pandemic.
But while A was closer to the virus found in bats, suggesting that human coronavirus came from that source and that AB spawned, it was B that was far more present in the marketplace.
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 AB gave birth.
“Otherwise, lineage A would have had to evolve in slow motion compared to lineage B virus, which just doesn’t make biological sense,” Worobey said.
Instead, the likely scenario was transferred from animals in the market to humans on various occasions in November and December 2019. The researchers concluded that it is unlikely that there was a human circulation before November 2019.
In this scenario, there were likely other animal-to-human transmissions in the market that did not manifest as Covid cases.
“Have we disproved the lab leak theory? No we have not. Will we ever be able to 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 likely.”
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