The researchers determined that the first cases of Covid-19 were centered in the market among vendors who sold these live animals or people who shopped there. They think there were two separate viruses circulating in animals that spread to humans.
“The eight cases of COVID-19 detected before December 20 were from the west side of the market, where mammalian 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 the Department of Immunology and Microbiology at Scripps Research, said Tuesday.
The “extraordinary” pattern that emerged from mapping these cases was very clear, said another co-author, Michael Worobey, head of the department of ecology and evolutionary biology at the University of Arizona.
Researchers mapped the first cases that had no connection to the market, Worobey noted, and those people lived or worked near the market.
“This indicates that the virus started to spread among people working at the market, but then started to spread … in the surrounding local community as vendors went to local shops, infected people working in these stores,” Worobey said.
The first version of the coronavirus, according to this research, likely came in different forms that scientists call A and B. The lineages were the result of at least two cross-species transmission events in humans.
The researchers suggest that the first animal-to-human transmission likely occurred around November 18, 2019, and was from lineage B. They only found lineage B type in people which had a direct link with the Huanan market.
The authors believe lineage A was introduced into humans from an animal within weeks or even days of infection with lineage B. Lineage A was found in samples from humans who lived or were staying near the market.
“These results indicate that SARS-CoV-2 is unlikely to have circulated widely in humans before November 2019 and define the narrow window between when SARS-CoV-2 first jumped in humans. man and when the first cases of COVID-19 were reported,” the study said. “As with other coronaviruses, the emergence of SARS-CoV-2 likely resulted from multiple zoonotic events.”
The likelihood of such a virus emerging from two different events is low, acknowledged co-author Joel Wertheim, assistant professor of medicine at the University of California, San Diego.
“Now I realize it sounds like I just said a once in a generation event has happened twice in a row, and pandemics are indeed rare, but once all the conditions are in place – it is a zoonotic virus capable of both human infection and human transmission in close proximity to humans – spillover barriers have been lowered so that multiple introductions in our view should in fact be expected,” Wertheim said.
Andersen said the studies don’t definitively refute the lab leak theory but are extremely compelling, 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 looked at it very carefully and looked at it more closely,” Andersen said. “Based on the data and analysis I’ve done over the past decade on many other viruses, I’ve convinced myself that the data does in fact point to this particular market.”
Worobey said he too believed the lab leak was possible, but the epidemiological preponderance of market-related cases is “not a mirage”.
“It’s a real thing,” he said. “It is simply not plausible that this virus was introduced other than through the wildlife trade.”
To reduce the risk of future pandemics, researchers hope to be able to determine exactly which animal may have been infected first and how.
“The raw ingredients for a zoonotic virus with pandemic potential are still lurking in nature,” Wertheim said. He thinks the world needs to do a much better job of monitoring and monitoring animals and other potential threats to human health.
Andersen said that while we can’t prevent epidemics, collaboration between the world’s scientists could be the key to the difference between a low-impact disease and one that kills millions.
“The big question we have to ask ourselves is this: the next time this happens, because it will happen, how do we go from detecting this epidemic early to preventing this epidemic so that it does not become a pandemic?