Swabs collected from stalls and equipment at the Huanan Market in Wuhan, China, that tested positive for traces of the virus that causes Covid-19 also, in some cases, contained traces of DNA from animals known to be susceptible to infection, according to a new analysis by an international team of researchers.
In the analysis, posted online Tuesday, the researchers say their findings add to evidence that live animals traded at the market could have passed the coronavirus to humans, pointing to a natural origin for the Covid-19 pandemic.
The swabs were collected by scientists affiliated with China’s Centers for Disease Control in early 2020, shortly after China closed the market, which was an early hot spot for Covid-19 infections.
No one had seen the raw data from that testing until Chinese researchers quietly uploaded details from their sampling to the genetic data-sharing platform GISAID.
The data was noticed by an international collaboration of researchers probing the pandemic’s origins in early March and downloaded for further study.
The new analysis drew attention last week after several media reports characterized its findings and World Health Organization officials discussed them in a news briefing. The analysis had not been made public until Tuesday, when it was posted online as a preprint study, ahead of peer review.
The species included in the analysis included raccoon dogs as well as red foxes, rabbits, cats and dogs. Genetic material from other mammals was also identified, including Amur hedgehogs, Malayan porcupines, hoary bamboo rats, Himalayan marmots, masked palm civets, Siberian weasels and hog badgers.
Scientists don’t know whether the virus that causes Covid-19 can infect every species they identified in the samples. Raccoon dogs are of particular interest because other studies have shown that they can be infected, they shed a lot of virus when they get sick, and they can transmit the infection to uninfected animals.
What’s more, most of the positive samples were concentrated in the southwest corner of the Huanan Market, an area that had previously been identified as an apparent hot spot for infections and an area where live animals were thought to have been sold.
The findings aren’t able to prove that any of these animals were infected with SARS-CoV-2, the virus that causes Covid-19, or that any of these animals infected people. Indeed, there were a few positive specimens from other parts of the market that contained only human DNA, suggesting that humans were shedding the virus there.
In addition to support for natural origin, the findings support the theory that there wasn’t a single spillover event from animals to humans but probably several, centered at the market, that eventually launched Covid-19 into the wider world.
Still, the researchers say their conclusions are limited because they were able to work with only a partial set of uploaded data.
Scientists involved with the reanalysis say they first noticed the data on March 4 and only realized how important it was on March 9, when they discovered that it corresponded to environmental samples taken at the Huanan Market near the start of the pandemic.
Chinese scientists posted their own analysis of the sampling as a preprint study in February 2022 but did not mention genetic sequences from non-human animals in the environmental swabs. That paper concluded that the busy Huanan Market had acted as an amplifier of early Covid-19 transmission but was probably not its source.
Western scientists have speculated that this information was deliberately left out of the Chinese preprint to avoid drawing attention to China’s controversial wild animal markets, which were known to spread respiratory diseases such as RSV and influenza. Scientists had cautioned for more than a decade that these markets had the potential to start pandemics, and before the Covid-19 pandemic, the Chinese government had done little to rein in this type of trade.
China closed the Huanan Market in early January 2020 and banned the trade and consumption of certain protected wild animals.
The new analysis emerged as Republicans in Congress have opened investigations into the pandemic’s origin. Previous studies provided evidence that the virus probably emerged naturally in market but could not point to a specific origin. Some US agencies, including a recent US Department of Energy assessment, say the pandemic probably resulted from a lab leak in Wuhan.
In a news briefing on Friday, WHO experts said that the data is not conclusive. They still can’t say whether the virus leaked from a lab or spilled over naturally from animals to humans.
“These data do not provide a definitive answer to the question of how the pandemic began, but every piece of data is important in moving us closer to that answer,” WHO Director-General Tedros Adhanom Ghebreyesus said.
He also accused China of withholding information pertinent to WHO’s investigation of the pandemic’s origins.
“This data could have, and should have, been shared three years ago,” Tedros said. “We continue to call on China to be transparent in sharing data and to conduct the necessary investigations and share results.”