Researchers determined that early cases of Covid-19 were centered around the market, among vendors who sold these live animals or among people who shopped there. They believe two separate viruses spread from animals to humans.
“All eight cases of COVID-19 detected before December 20 came from the western part of the market, where mammal species were also sold,” the study said. Proximity to five stalls selling live or recently butchered animals predicts human events.
“The clustering is very specific,” study co-author Christian Anderson, a professor in the Department of Immunology and Microbiology at Scripps Research, said Tuesday.
The “extraordinary” pattern that emerged from mapping these events was very clear, said another co-author, Michael Worobe, chair of the Department of Environmental and Evolutionary Biology at the University of Arizona.
The researchers mapped early cases that had no connection to the market, Worobe noted, and those people lived or worked in close proximity to the market.
“It started spreading the virus to people who worked in the market, but then it started to spread … to the surrounding local community as vendors went to local shops and people who worked in those shops became infected,” Worobe said.
The early version of the coronavirus, this research shows, may have different forms that scientists call A and B. Inheritance is the result of at least two cross-species human transmission events.
As of November 18, 2019, animal-to-human transmission may have occurred, and the researchers report that it originated from the B lineage. They found lineage B only in people who had direct contact with the Huanan market.
The authors believe that lineage A was introduced to humans from an animal within weeks or days of infection from lineage B. Lineage A was detected in samples from humans who lived or stayed near the market.
“These findings indicate that SARS-CoV-2 was unlikely to have spread widely among humans before November 2019 and define a narrow window between when SARS-CoV-2 first jumped into humans and when the first cases of COVID-19 were reported,” the study says. “Like other coronaviruses, the origin of SARS-CoV-2 may be the result of multiple zoonotic events.”
The chances of such a virus emerging from two separate events are slim, agreed co-author Joel Wertheim, an associate professor of medicine at the University of California, San Diego.
“Now, a once-in-a-generation event has happened twice in a short period of time, and epidemics are really rare, but once all the conditions are in place — it’s a zoonotic virus. Both human infection in close proximity to humans and human transmission — the barriers to spillover are lowered, meaning We believe that many introductions are indeed to be expected,” said Wertheim.
Anderson said the studies didn’t conclusively prove the lab leak theory, but they were so compelling that he changed his mind about the origin of the virus.
“I was pretty sure it was a lab leak until we looked at it very carefully and looked at it very closely,” Anderson said. “Based on the data and analysis I’ve done on many viruses over the last decade, I’m convinced that the data really points to this particular market.”
Worobe said he also thought a lab leak was possible, but the epidemiological preponderance of cases linked to the market was “not a miracle.”
“It’s a real thing,” he said. “It is not plausible that the virus was introduced through the wildlife trade in any other way.”
Researchers hope to be able to determine precisely which animal may have been infected first, and how, to reduce the chances of future outbreaks.
“The raw materials for zoonotic viruses with infectious potential are still lurking in the wild,” Wertheim said. He believes the world needs to do a better job of monitoring and monitoring other potential threats to animal and human health.
Andersen said that while we can’t stop outbreaks, collaboration between the world’s scientists is key to the difference between a disease that has a small impact and one that kills millions.
“The big question we have to ask ourselves is — the next time this happens, because it will — how do we detect that outbreak early and prevent that outbreak from becoming an epidemic?”
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