According to a pair of related studies, 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.
The findings, made by an international team led by the University of California San Diego, may call into question the theory that the virus leaked from the infamous Wuhan Institute of Virology, causing the pandemic that has claimed at least 6.4 million lives.
Instead, the studies, which were published online in the journal Science via First Release, demonstrated that a pathogen successfully jumped from a non-human animal host to a human in what is known as a zoonotic event.
It is proposed that the SARS-CoV-2 virus jumped from animals to humans at least twice, and possibly up to a 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 the researchers, two evolutionary branches of the virus were present early in the pandemic, distinguished only by two nucleotide differences – the basic building blocks of DNA and RNA.
Lineage B, which included samples from market workers and visitors, rose to global dominance. Lineage A spread throughout China and included samples from people only found in the vicinity of the market.
Wertheim believes SARS-CoV-2 jumped from animals to humans only once if the viruses in lineage A evolved from those in lineage B.
However, the researchers discovered that the earliest SARS-CoV-2 genomes were incompatible with a single zoonotic jump into humans. Rather, the first zoonotic transmission with lineage B viruses occurred in late November 2019, with the introduction of lineage A into humans occurring within weeks of the first event. Both strains were on the market at the same time.
The researchers arrived at this conclusion by analysing the evolutionary rate of viral genomes to determine whether the two lineages diverged from a single common ancestor in humans.
The findings provide compelling evidence that the two viral lineages evolved independently and that multiple spillover events occurred.
The Wuhan market reportedly had a thriving live wild animal trade, with snakes, badgers, muskrats, birds, and raccoon dogs (an Asian canid) and other species sold for food. Wertheim believes there were numerous viral introductions. At least two viral strains successfully crossed the animal-human divide; other viral strains became 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.”
