The researchers determined that the first cases of Covid-19 were concentrated in the market among sellers who sold these live animals or people who shop there. They believe that there were two separate viruses circulating in animals that were transmitted to humans.
“All 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. The proximity of five stalls selling live or recently slaughtered animals was a predictor of human cases.
“The clustering is very, very specific,” study co-author Kristian Andersen, a professor in the Department of Immunology and Microbiology at the Scripps Research Institute, said Tuesday.
Another co-author, Michael Worby, chair of the Department of Ecology and Evolutionary Biology at the University of Arizona, said the “unusual” pattern that emerged from the mapping of these cases was very evident.
Worby noted that the researchers mapped the early cases that had nothing to do with the market, and those people lived or worked near the market.
“This is an indication that the virus started to spread among people who worked in the market, but then it started spreading…in the surrounding community where the sellers went to the local stores, and they infected the people who worked in those stores,” Worubi said.
This research shows that the first version of the coronavirus likely came in different forms that scientists call A and B. The strains were the result of 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 type B only in people with a direct link to the Huanan market.
The authors believe that lineage A was introduced to humans from an animal within weeks or even days of infection from lineage B. Lineage A has been found in samples from humans who lived or remained near the market.
“These results indicate that SARS-CoV-2 is unlikely to have spread widely in humans prior to November 2019 and define the narrow window between when SARS-CoV-2 first jumped into humans and the time when the first cases of SARS-CoV-2 were reported. COVID-19,” the study says. “As with other coronaviruses, SARS-CoV-2 likely emerged from multiple animal events.”
Co-author Joel Wertheim, an assistant professor of medicine at the University of California, San Diego, acknowledged that the probability of such a virus emerging from two different events is low.
“Now, I realize it’s as if you just said that an event that repeats once in every generation happened twice in short succession, and that pandemics are really rare, but once all the conditions are created – that’s an animal virus that is capable of human infection and human transmission,” Werthem said. Too close to humans – the barriers to infection spread have been lowered so that we think multiple precursors should be expected.”
Andersen said the studies don’t conclusively disprove the lab leak theory but are so compelling, that he changed his mind about the origins of the virus.
“I was absolutely convinced the lab leaked myself, until we delved into this very carefully and looked closely at it,” Andersen said. “Based on data and analyzes I have done over the past decade on many other viruses, I have convinced myself that these are in fact the data points for this particular market.”
Worby said he also believed leaks in the lab were possible, but the epidemiological predominance of market-related cases “is not a mirage.”
“It’s a real thing,” he said. “It is not reasonable for this virus to be introduced in any way other than the wildlife trade.”
To reduce the chances of future epidemics, the researchers hope to be able to determine which animal might have been infected first and how it was infected.
“The raw ingredients of a zoonotic virus with pandemic potential are still latent in the wild,” Werthem 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 although we can’t prevent outbreaks, collaboration among the world’s scientists could be the key to the difference between a disease with little impact and one that kills millions.
“The big question we need to ask ourselves is – the next time this happens, because it will happen – how do we go from detecting this outbreak early and preventing this outbreak so that it doesn’t become a pandemic?”