(CNN) — In June, the World Health Organization (WHO) recommended that scientists continue to investigate all possible origins of the covid-19 pandemic, including a laboratory leak. Two recently published studies take totally different approaches but come to the same conclusion: The Huanan Seafood Market in Wuhan, China was likely the epicenter of the coronavirus.
The studies were published online as preprints in February, but have now undergone peer review and were published Tuesday in the journal Science.
In one, scientists around the world used social media reporting and mapping tools to conduct spatial and environmental analysis. They suggest that although the “exact circumstances remain obscure”, the virus was likely present in live animals sold on the market in late 2019. The animals were kept close together and could have easily exchanged germs. However, the study does not determine which animals may have been sick.
The researchers determined that the first cases of covid-19 were concentrated in the market among the vendors who sold these live animals or the people who bought there. They believe there were two separate viruses circulating in the animals that spread to people.
“The eight cases of covid-19 detected before December 20 were from the western side of the market, where mammalian species were also sold,” the study says. Proximity to five stalls selling live or recently slaughtered animals predicted human cases.
“The clustering is very, very specific,” study co-author Kristian Andersen, a professor in the Department of Immunology and Microbiology at Scripps Research, said at a news conference Tuesday.
The “extraordinary” pattern that emerged from mapping these cases was very clear, said another co-author, Michael Worobey, chair of the Department of Ecology and Evolutionary Biology at the University of Arizona.
The researchers mapped the first cases that had no connection to the market, Worobey noted, and those people lived or worked in close proximity to the market.
“This is an indication that the virus started to spread in people working in the market, but then started to spread…to the surrounding local community when vendors entered local stores, infecting people working in those stores. Worobey said.
The first animal-to-human transmission
the other studio takes a molecular approach and appears to determine when the first coronavirus infections crossed from animals to humans.
The oldest version of the coronavirus, this research shows, likely came in different forms that scientists call A and B. The lineages were the result of at least two cross-species transmission events to humans.
The researchers suggest that the first animal-to-human transmission likely occurred around November 18, 2019, and came from the B lineage. They found the B lineage type only in people who had a direct connection to the Huanan market.
The authors believe that lineage A was introduced into humans from an animal within a few weeks or even days of infection with lineage B. Lineage A was found in samples from humans who lived or stayed near the market.
“These findings indicate that SARS-CoV-2 is unlikely to have circulated widely among humans before November 2019 and define the narrow window between the time SARS-CoV-2 first jumped to humans and the when the first cases of covid-19 were reported,” the study says. “As with other coronaviruses, the emergence of SARS-CoV-2 was likely the result of multiple zoonotic events.”
The likelihood of such a virus arising from two different events is low, acknowledged co-author Joel Wertheim, an associate assistant professor of medicine at the University of California, San Diego.
“Now, I realize it sounds like I just said that a once-in-a-generation event happened twice in short succession, and pandemics are certainly rare, but once all the conditions are met—a zoonotic virus capable of both of infection and of human transmission that is very close to humans—the barriers to contagion have been lowered to such an extent that we think multiple introductions should actually be expected,” Wertheim said.
Andersen said the studies don’t definitively disprove the lab leak theory, but they are extremely persuasive, so much so that he changed his mind about the origins of the virus.
“I was pretty convinced of the lab leak myself, until we dove into this very carefully and looked at it much more closely,” Andersen said. “Based on the data and analysis I’ve done over the last decade on many other viruses, I’ve become convinced that the data actually points to this particular market.”
Worobey said he, too, thought the lab leak was possible, but that the epidemiological preponderance of market-linked cases “is not a mirage.”
“It’s a real thing,” he said. “It is simply not plausible that this virus was introduced in any way other than through the wildlife trade.”