Humans may have brought Covid-19 to the Wuhan market where the virus first emerged, scientists have said, after finding no proof that raccoon dogs were to blame for the outbreak.
It comes after a controversial study suggested last month that raccoon dog DNA found at the Huanan Seafood Market in January 2020 provided “strong evidence” that the virus was transmitted to humans at the site.
The paper was based on samples taken by Chinese researchers at the start of the pandemic which had been uploaded to an international genetic database before they had published their own analysis of the data.
But on Wednesday, the Chinese scientists released their study in the journal Nature and said there was no way of knowing if the raccoon dogs were infected. They also cautioned that the origins of Covid-19 could not be determined from their samples.
Writing in the journal, the authors, which include George Gao, the former head of the Chinese Center for Disease Control and Prevention (DCD), said: “These environmental samples cannot prove that the animals were infected.
“Furthermore, even if the animals were infected, our study does not rule out that human-to-animal transmission occurred, considering the sampling time was after the human infection within the market as reported retrospectively.
“Thus, the possibility of potential introduction of the virus to the market through infected humans, or cold chain products, cannot be ruled out yet.”
The Huanan Seafood Market was associated with a cluster of early cases, which has led some scientists to suggest that Covid-19 may have jumped to a human from an animal.
Raccoon dogs were thought to be a likely candidate for the initial spread because they are known to be susceptible to the virus.
But the researchers pointed out that humans had already been infected by the time they first took swabs in January 2020, so even if the animals were infected they could have caught the virus from humans.
The team also found traces of Covid-19 in sewers, suggesting that infected humans or animals may have helped spread the virus. Previous studies have pointed out that the largest concentration of Covid-19 was found near the market’s toilets.
Pandemic's origins 'unlikely to be Wuhan market'
Viscount Ridley, author of the book Viral: The Search For The Origin of Covid-19, said that maps of the collection areas showed there was little overlap between viral DNA and raccoon dog DNA.
“Not only were no raccoon dogs found to be infected, they were mostly in the wrong part of the market,” he said.
“The scientists who investigated the Huanan seafood market in Wuhan in 2020 have concluded that the pandemic probably did not begin there.
“To believe it did, you have to assume that they missed or covered up some infected animals.”
The earlier study, led by Kristian Andersen, Edward Holmes, and Michael Worobey, which had not been submitted for peer-review, was dismissed as “risibly thin” and misleading by some scientists.
In a pre-print last year, many of the same authors claimed to have “incontrovertible evidence” that Covid-19 emerged from the wildlife trade, but later rowed back on the assertions.
The experts were also criticised by the Gisaid database for taking the samples from another research group and submitting their own study while the Chinese scientists were going through a lengthy peer-review process.
Before publication, the story was also leaked to The Atlantic, who claimed it was "the strongest evidence yet that an animal started the pandemic."
However when they published their work, the authors acknowledged that animal DNA in a sample did not mean that animal was the source of the virus.
“The most abundant animal in the sequencing data of a particular sample is not necessarily the source of the virus in that sample,” they wrote.
The Chinese team said that “internationally coordinated efforts” were needed to investigate the origins of the pandemic.
The World Health Organisation (WHO) has accused China of being obstructive in its efforts to determine whether the virus could have leaked from a lab.
The Huanan Seafood Market was just a few miles from the Wuhan Institute of Virology (WIV) which was collecting and carrying out experiments on bat coronaviruses - the closest relatives to Covid-19.