In one, scientists from around the world used social network reporting and mapping tools to do spatial and environmental analysis. They suggest that while the “exact circumstances remain murky,” the virus was likely present in live animals sold on the market in late 2019. The animals were living 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 centered in the market among vendors who sold these live animals or people who bought from them. They believe there were two separate viruses circulating in the animals that spilled into people.
“All eight cases of COVID-19 detected before December 20 were from the west side of the market, where mammal species were also sold,” the study says. Proximity to five stalls selling live or recently butchered 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.
The researchers mapped the first cases that had no connection to the market, Worobey noted, and those people lived or worked very close to the market.
“This is an indication that the virus started to spread among people working in the market, but then it started to spread … into the surrounding local community as vendors went into local shops, people infected people who worked in these stores,” Worobey said.
The other study takes a molecular approach and looks to determine when the first coronavirus infections passed from animals to humans.
The earliest version of the coronavirus, this research shows, likely had different forms that scientists call A and B. The lineages resulted from at least two cross-species transmission events to humans.
The researchers suggest that the first animal-to-human transmission probably occurred around November 18, 2019, and came from lineage B. They found lineage type B only in people who had a direct connection to the market. Huanan
The authors believe that lineage A was introduced into humans from an animal a few weeks or even days after infection with lineage B. Lineage A was found in samples from humans who lived or stayed near the market.
“These findings indicate that it is unlikely that SARS-CoV-2 was widely circulating in humans before November 2019 and define the narrow window between when SARS-CoV-2 first jumped into humans and when the first cases of COVID-19,” the study says. “As with other coronaviruses, the emergence of SARS-CoV-2 was likely the result of multiple zoonotic events.”
The likelihood of this virus arising from two separate events is low, acknowledged co-author Joel Wertheim, an assistant 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 short succession, and pandemics are really rare, but once all the conditions are met, this is a virus zoonotic capable of both human infection and human transmission that is in close proximity to humans—the barriers to spread have been reduced so that we believe multiple introductions should be expected,” Wertheim said.
Andersen said the studies do not definitively disprove the lab leak theory, but are extremely persuasive, so much so that he changed his mind about the origins of the virus.
“I was pretty convinced about the leak from the lab, until we got into it very carefully and looked at it much more closely,” Andersen said. “Based on data and analysis I’ve done over the last decade on many other viruses, I’m convinced that the data really points to this particular market.”
Worobey said he, too, thought laboratory leakage was possible, but the epidemiological preponderance of market-linked cases “is not a mirage.”
“It’s a real thing,” he said. “It is not plausible that this virus was introduced in any way other than through the wildlife trade.”
To reduce the chances of future pandemics, researchers hope to be able to determine exactly which animal may have been infected first and how.
“The raw materials for a zoonotic virus with pandemic potential are still lurking in nature,” Wertheim said. He believes the world needs to do a much better job of monitoring and controlling animals and other potential threats to human health.
Andersen said that while we can’t prevent outbreaks, collaboration among the world’s scientists could be key to differentiating a disease with a small impact from one that kills millions.
“The big question we have to ask ourselves is: The next time this happens, because it will, how can we detect this outbreak early and prevent it from becoming a pandemic?”