The SARS-CoV-2 virus, which cause COVID-19, originated in animals and spread to humans in late 2019 at the Huanan Seafood Market in Wuhan, China, two new studies suggest.
The first study used spatial analysis to show that the earliest known COVID-19 cases, diagnosed in December 2019, were centered on the Wuhan market.
The researchers found that environmental samples that tested positive for the virus were strongly associated with the vendors selling live animals.
"We used the maps in the World Health Organisation mission's report on the origin of SARS-CoV-2 to extract latitude and longitude for most of the known COVID-19 cases from Wuhan with symptom onset in December 2019," Michael Worobey, a professor at the University of Arizona, US, tweeted.
"We found that cases in December were both nearer to, and more centered on, the Huanan market than could be expected. Its epicenter was at the market," said Worobey, who was the author on the both the research papers.
A second study found that the two major viral lineages were the result of at least two events in which the virus jumped from animals to humans.
The first transmission most likely happened in late November or early December 2019, and the other lineage was probably introduced within weeks of the first event, the researchers said.
These findings define the narrow window between when SARS-CoV-2 first jumped into humans and when the first cases of COVID-19 were reported, they said.
"As with SARS-CoV-1 in 2002 and 2003, SARS-CoV-2 emergence likely resulted from multiple zoonotic events," the authors wrote in the second research paper.
The yet-to-be peer-reviewed studies, however, could not identify an animal at the market that spread the virus to humans.
"We show that, contrary to what some may have believed, wished to believe or asserted, live mammals susceptible to SARS-CoV-2 were present at the Huanan market in the crucial months of November and December 2019," he added.
The findings support a report published last year in The lancet journal which was compiled by two dozen biologists, ecologists, epidemiologists, physicians, public health experts and veterinarians from around the world.
The report had concluded that there was no scientifically validated evidence to support the theory that the coronavirus leaked from a laboratory in China, and that peer-reviewed studies strongly suggest that the virus evolved in nature.