New York, July 27 The origin of the Covid-19 pandemic in 2019 was at the Huanan Seafood Wholesale Market in Wuhan, China, and resulted from multiple spillover events of the SARS-CoV-2 virus jumping from live animal hosts to humans working or shopping there, according to a pair of related studies.
The findings, by a team of international researchers led by the team at University of California San Diego, may quash the theory that the virus leaked from the lab at the infamous Wuhan Institute of Virology causing the pandemic that has so far claimed at least 6.4 million lives.
Instead, the studies, published online via First Release in the journal Science, showed that a pathogen successfully jumped from a non-human animal host to human, in what is known as a zoonotic event.
It posits that the SARS-CoV-2 virus jumped from animals to humans at least twice and perhaps as many as two dozen times.
"It's vital that we know as much about the origin of Covid-19 as possible because only by understanding how pandemics get started can we hope to prevent them in the future," said Joel O. Wertheim, Associate Professor in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine.
"I think there's been consensus that this virus did in fact come from the Huanan Market, but a strong case for multiple introductions hasn't been made by anyone else yet," said Wertheim.
According to researchers, two evolutionary branches of the virus were present early in the pandemic, differentiated only by two differences in nucleotides the basic building blocks of DNA and RNA.
Lineage B, which included samples from people who worked at and visited the market, became globally dominant. Lineage A spread within China, and included samples from people pinpointed only to the vicinity of the market.
If the viruses in lineage A evolved from those in lineage B, or vice versa, Wertheim said this would suggest SARS-CoV-2 jumped only once from animals to humans.
But the study found that the earliest SARS-CoV-2 genomes were inconsistent with a single zoonotic jump into humans. Rather, the first zoonotic transmission likely occurred with lineage B viruses in late-November 2019 while the introduction of lineage A into humans likely occurred within weeks of the first event. Both strains were present at the market simultaneously.
Researchers arrived at this conclusion by deciphering the evolutionary rate of viral genomes to deduce whether or not the two lineages diverged from a single common ancestor in humans.
The data are powerful evidence that the two viral lineages evolved separately and that multiple spillover events occurred.
The Wuhan market reportedly contained a robust live wild animal business, with snakes, badgers, muskrats, birds and raccoon dogs (a canid indigenous to Asia) and other species sold for food. Wertheim said he believes there were likely many viral introductions. At least two successfully made the animal-human leap; other viral strains went extinct.
"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," Wertheim said. "There are really no other good explanations for both of these strains being at the market except for multiple jumps into humans."
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