The researchers found that the earliest Covid-19 cases centered on the market among vendors who sold these live animals or people who shopped there. They believe there were two separate viruses circulating in the animals that passed into humans.
“All eight COVID-19 cases detected before December 20 came from the western end of the market, where mammal species were also sold,” the study said. The proximity of five stalls selling live or recently slaughtered animals was predictive of human cases.
“The clustering is very, very specific,” study co-author Kristian Andersen, a professor in Scripps Research’s Department of Immunology and Microbiology, said Tuesday.
The “extraordinary” pattern that emerged from the mapping of these cases was very clear, said another co-author, Michael Worobey, department chief of Ecology and Evolutionary Biology at the University of Arizona.
The researchers identified the earliest cases that were unrelated to the market, Worobey noted, and those people lived or worked near the market.
“This is an indication that the virus started spreading among people who worked in the market, but then spread … to the surrounding local community as vendors entered local stores, infected people who worked in those stores,” Worobey said.
The earliest version of the coronavirus, this research shows, likely came in different forms that the scientists call A and B. The lineages were the result of at least two cross-species transfer events to humans.
The researchers suggest that the first animal-to-human transmission likely occurred around November 18, 2019, and came from line B. They found the line B type only in people who had a direct connection to the Huanan market.
The authors believe that lineage A was introduced into humans by an animal within weeks or even days after the infection of lineage B. Lineage A was found in samples from people who lived or stayed near the market.
“These findings indicate that SARS-CoV-2 is unlikely to be widely circulating in humans prior to November 2019 and define the narrow window between when SARS-CoV-2 first jumped into humans and when the first cases of COVID-19 were reported,” the study says. “As with other coronaviruses, the emergence of SARS-CoV-2 is likely the result of multiple zoonotic events.”
The chances of such a virus arising from two different events are slim, acknowledged co-author Joel Wertheim, an associate 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 quick succession, and pandemics are indeed rare, but once all the conditions are met, that’s a zoonotic virus that is capable of both human infection and human transmission close to humans — the barriers to overflow have been lowered so that multiple introductions, in our view, should actually be expected,” Wertheim said.
Andersen said the studies don’t definitively disprove the lab leak theory, but are extremely convincing, so much so that he changed his mind about the origin of the virus.
“I was pretty convinced about the lab leak myself, until we dug into this very carefully and looked at it much more closely,” Andersen said. “Based on data and analysis I’ve done on many other viruses over the past ten years, I’ve convinced myself that the data actually refers to this particular market.”
Worobey also said he thought the lab leak was possible, but the epidemiological preponderance of market-related cases is “not a mirage.”
“It’s real,” he said. “It’s just not likely that this virus got in any other way than through wildlife trade.”
To reduce the chance of future pandemics, the researchers hope to determine exactly which animal became infected first and how.
“The raw materials for a zoonotic virus with pandemic potential are still lurking in the wild,” Wertheim said. He believes the world needs to do much better by guarding and monitoring animals and other potential threats to human health.
Andersen said that while we can’t prevent outbreaks, collaboration between scientists from around the world could be the key to the difference between a small-impact disease and one that costs millions.
“The big question we have to ask ourselves is — next time this happens, because it will happen — how can we detect that outbreak early and prevent that outbreak so it doesn’t become a pandemic?”