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Scientists have released three studies that reveal intriguing new clues about how the COVID-19 pandemic started. Two of the reports trace the outbreak back to a massive market that sold live animals, among other goods, in Wuhan, China and a third suggests that the coronavirus SARS-CoV-2 spilled over from animals—possibly those sold at the market—into humans at least twice in November or December 2019. Posted on 25 and 26 February, all three are preprints, and so have not been published in a peer-reviewed journal.

These analyses add weight to original suspicions that the pandemic began at the Huanan Seafood Wholesale Market, which many of the people who were infected earliest with SARS-CoV-2 had visited. The preprints contain genetic analyses of coronavirus samples collected from the market and from people infected in December 2019 and January 2020, as well as geolocation analyses connecting these samples to a section of the market where live animals were sold. Taken together, these different lines of evidence point towards the market as the source of the outbreak—much like animal markets were ground zero for the severe acute respiratory syndrome (SARS) epidemic of 2002–2004—says Kristian Andersen, a virologist at the Scripps Research Institute in La Jolla, California, and an author on two of the reports. “This is extremely strong evidence,” he says.

Still, none of the studies contain definitive evidence about what type of animal might have harbored the virus before it spread to humans. Andersen speculates that the culprits could be raccoon dogs, a squat dog-like mammal used for food and for their fur in China. One of the studies he coauthored suggests that raccoon dogs were sold in a section of the market where several positive samples were collected. And reports show that the animals are capable of harboring other types of coronaviruses.

Some virologists say that the new evidence pointing to the Huanan market doesn’t rule out an alternative hypothesis. Namely, they say that the market could have just been the location of a massive amplifying event, in which an infected person spread the virus to many other people, rather than the place of the original spillover.

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