In the latest volley of the debate over the origins of the coronavirus, a group of scientists this week presented a review of scientific findings that they argue shows a natural spillover from animal to human is a far more likely cause of the pandemic than a laboratory incident.
Among other things, the scientists point to a recent report showing that markets in Wuhan, China, had sold live animals susceptible to the virus, including palm civets and raccoon dogs, in the two years before the pandemic began.
They observed the striking similarity that Covid-19's emergence had to other viral diseases that arose through natural spillovers and pointed to a variety of newly discovered viruses in animals that are closely related to the one that caused the new pandemic.
The back and forth among scientists is taking place while intelligence agencies are working with an end-of-August deadline to provide US President Joe Biden with an assessment of the origin of the pandemic. There is now a division among intelligence officials as to which scenario for viral origin is more likely.
The new paper, which was posted online on Wednesday (Thursday NZT) but has yet to be published in a scientific journal, was written by a team of 21 virus experts. Four of them also collaborated on a 2020 paper in Nature Medicine that largely dismissed the possibility that the virus became a human pathogen through laboratory manipulation.
In the new paper, the scientists provided more evidence in favour of the virus having spilled over from an animal host outside of a laboratory.
Joel Wertheim, a virus expert at the University of California, San Diego, and a co-author, said that an important point in support of a natural origin was the "uncanny similarity" between the Covid and severe acute respiratory syndrome pandemics.
Both viruses emerged in China in the late autumn, he said, with the first known cases popping up near animal markets in cities — Wuhan in the case of Covid, and Shenzen in the case of SARS.
In the SARS epidemic, the new paper points out, scientists eventually traced the origin to viruses that infected bats far from Shenzen.
Based on the distribution of viruses similar to the new coronavirus across Asia, Wertheim and his colleagues predict the origin of SARS-CoV-2 will also be far from Wuhan.
Since first surfacing in the final months of 2019, this pandemic's viral culprit has yet to be found naturally occurring in any animal.
In May, another team of 18 scientists published a letter arguing that the possibility of a lab leak needed to be taken seriously because there was too little evidence to favour a natural origin of the coronavirus or a leak from a lab.
Wuhan, where the pandemic was first documented, is home to the Wuhan Institute of Virology, or WIV for short, where researchers have studied coronaviruses from bats for years.
One of the signers of the May 2021 letter, Michael Worobey of the University of Arizona, became a co-author of the new paper arguing for a natural spillover.
He said his views have evolved as more information emerges. Among other reasons for Worobey's shift was the growing evidence about the Huanan animal market in Wuhan.
When the pandemic first arose in Wuhan, Chinese officials tested hundreds of samples from animals sold at the market and did not find the coronavirus in any of them.
But last month a team of researchers presented an inventory of 47,381 animals from 38 species sold in Wuhan markets between May 2017 and November 2019. It included species like civets and raccoon dogs that can act as intermediate hosts for coronaviruses.
Worobey called that study "a game-changing paper".
He also pointed to the timing of the earliest cases of Covid in Wuhan.
"The Huanan market is right at the epicentre of the outbreak, with later cases then radiating outward in space from there," Worobey said.
"No early cases cluster anywhere near the WIV, which has been the focus of most speculation about a possible lab escape."
Other scientists, however, say that such arguments are speculative and that the new review is mostly a rehash of what was already known.
"Basically, it really boils down to an argument that because nearly all previous pandemics were of natural origin, this one must be as well," said David Relman, a microbiologist at Stanford University who organised the May letter to Science.
He noted that he does not object to the natural origin hypothesis as a plausible explanation for the pandemic origin. But Relman thinks the new paper presented "a selective sampling of findings to argue one side".
Worobey and his colleagues also presented evidence in their new paper against the idea that so-called gain-of-function research that intentionally alters the function of a virus might have played a role in the pandemic. The researchers argue that the genome of the coronavirus shows no compelling signatures of being manipulated. And the diversity of coronaviruses scientists have been discovering in Asian bats could have served as the evolutionary wellspring for Covid-19.
But Richard Ebright, a molecular biologist at Rutgers University and a persistent critic of attempts to diminish the likelihood of a laboratory leak, said that this was a straw-man argument.
Ebright said it was possible that a WIV lab worker might have contracted the coronavirus on a field expedition to study bats or while processing a virus at the lab. The new paper, he argued, failed to address such possibilities.
"The review does not advance the discussion," Ebright said.
Written by: Carl Zimmer and James Gorman
© 2021 THE NEW YORK TIMES