Markets in Wuhan, China, sell fresh meat in the open air, often offering live animals for slaughter. File Photo by Stephen Shaver/UPI | License Photo
Two new studies strongly suggest that COVID-19 most likely began with a jump to humans from animals sold at the Huanan Seafood Market in Wuhan, China.
It's not clear from what type of animal the virus jumped to a human, but the first animal-to-human transmission appears to have happened around Nov. 18, 2019, one study found.
According to the researchers, the Wuhan market contained a wide range of live wild animals, with badgers, birds, muskrats, snakes and other species sold for food.
Two strains of SARS-CoV-2, the virus that causes COVID-19, already were present at the market by the fall of 2019, the studies found.
"While I'm hesitant to call it proof, what we presented is the most comprehensive explanation for the SARS-CoV-2 genomic diversity at the outset of the pandemic," study co-author Joel Wertheim, an associate adjunct professor of medicine at the University of California, San Diego, said in a university news release.
"There are really no other good explanations for both of these strains being at the market except for multiple jumps into humans."
The two studies first were posted online in February as preprints, but they've now undergone peer review and were published Tuesday in the journal Science.
Other theories about how the virus began in humans include the "Chinese lab leak" hypothesis widely circulated on the internet. The World Health Organization has recommended that scientists continue research on that and other theories.
But the researchers behind the two new studies believe their findings deliver convincing evidence on the virus' origins.
"It's a real thing," said Michael Worobey, a co-author of the first study and department head of Ecology and Evolutionary Biology at the University of Arizona, told CNN. "It's just not plausible that this virus was introduced any other way than through the wildlife trade."
In one of the two studies, researchers conducted spatial and environmental analysis using mapping tools and social media reports to track the emergence and spread of the novel coronavirus. In the other study, the research team took a molecular approach. Yet, both arrived at the same conclusion.
Researchers in the first study determined that the earliest COVID-19 cases occurred among Huanan Seafood Market vendors who sold live animals or the people who shopped there.
The research team believes that two separate viral strains began circulating in animals that then infected people.
"All eight COVID-19 cases detected prior to 20 December were from the western side of the market, where mammal species were also sold," the study authors noted.
Worobey called the pattern extraordinary and very clear. Even the earliest cases not involving people who worked or shopped at the market occurred in people who worked near it or lived near it.
"This is an indication that the virus started spreading in people who worked at the market but then started that spread ... into the surrounding local community as vendors went into local shops, infected people who worked in those shops," Worobey said.
In the second study, researchers identified two viral strains they labeled the A and B lineages of the virus.
These resulted from at least two cross-species transmissions into humans, the research team said. The first animal-to-human jump was from lineage B around Nov. 18, 2019, and was found only in people who had a direct connection to the market. Lineage B later went on to become the globally dominant strain.
The authors believe that lineage A -- found in samples from humans who lived near the market -- was introduced into humans from an animal either days or weeks later.
"These findings indicate that it is unlikely that SARS-CoV-2 circulated widely 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 said.
"As with other coronaviruses, SARS-CoV-2 emergence likely resulted from multiple zoonotic [animal-to-human] events."
Researchers from both studies warned of the possibility of future outbreaks of new viral threats.
"This has happened before, and it's going to keep happening. Nature is a better lab than humans will ever be," Jonathan Pekar, a doctoral student in Bioinformatics and Systems Biology who co-led the project with Wertheim, said in the UCSD news release.
Andersen called for more collaboration between the world's scientists.
"The big question we need to ask ourselves," he said, "is -- the next time this happens, because it will happen -- how do we go from detecting that outbreak early and preventing that outbreak so it doesn't become a pandemic?"
The U.S. Centers for Disease Control and Prevention has more on COVID-19.
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