New research points to Wuhan market as origin of pandemic

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Scientists released a pair of in-depth studies on Saturday that point to a market in Wuhan, China as the source of the coronavirus pandemic. Analyzing data from various sources, they concluded that the coronavirus was most likely present in live mammals sold at the Huanan Seafood Wholesale Market in late 2019 and suggested that the virus had twice spread to people who worked there or shopped there. They said they found no support for an alternative theory that the coronavirus escaped from a lab in Wuhan.



Members of the Wuhan Hygiene Emergency Response Team leaving the closed Huanan Seafood Wholesale Market on January 11, 2020.

Noel Celis/Agence France-Presse

“When you look at all the evidence together, it’s an extraordinarily clear picture that the pandemic started in the Huanan market,” said Michael Worobeyevolutionary biologist at the University of Arizona and co-author of both studies.

Both reports have yet to be published in a scientific journal that would require peer review.

Together, they represent a major salvo in the debate about the beginnings of a pandemic that has killed nearly 6 million people worldwide and sickened more than 400 million. Whether the coronavirus outbreak began with an overflow of wild animals being sold in the market, a leak from a Wuhan virology lab, or otherwise has resulted in pitched geopolitical battles and debates about the best way to stop the next pandemic.

But some outside scientists who were hesitant to endorse the market origin hypothesis said they were still unconvinced. Jesse Bloom, a virologist at the Fred Hutchinson Cancer Research Center, said in an interview that there remained a glaring absence of direct evidence that animals in the market had themselves been infected with the coronavirus.

“I think what they’re arguing might be true,” Dr Bloom said of the new studies. “But I don’t think the quality of the data is good enough to say any of these scenarios are true with confidence.”

In their new study, Dr Worobey and colleagues present evidence that wild mammals that may have harbored the coronavirus were being sold in December 2019. But there were no wild animals left on the market by the time Chinese researchers arrived. in early 2020 to collect genetic samples. .


Pictures of animals for sale in the Huanan market.

Animals for sale in the Huanan market in 2019 and 2014, including raccoon dogs, Malayan porcupines and a red fox.

Source: Michael Worobey et al., preprint via Zenodo. Photos taken by a citizen and posted on Weibo in 2019 (first three), and by Edward C. Holmes in 2014.

The authors of the new study include researchers who had previously published smaller reports that had pointed to a similar conclusion, but were based on far fewer details. Their earlier analysis suggested that the first known case of coronavirus was a vendor in the Huanan market.

In a separate line of research, scientists from the Chinese Center for Disease Control and Prevention conducted a new analysis of genetic traces of coronavirus collected from the market in January 2020. Previous studies have shown that viruses taken from the first cases of Covid belonged to two main evolutionary branches. Samples from the Huanan market included both branches, the scientists reported in a study they posted online Friday.

Dr Worobey, who said he was unaware of the study until it was made public, said their findings are consistent with the scenario he and his colleagues have proposed for two origins in the market.

“The beauty of it all is how much it all adds up now,” said Jeremy Kamil, a virologist at Louisiana State University Health Sciences, who was not involved in the new study.


Map of Wuhan showing the location of Huanan Seafood Wholesale Market.




Huanan Seafood

wholesale market

Huanan Seafood

wholesale market


The Huanan Seafood Wholesale Market was one of the first objects of suspicion when Covid first swept through Wuhan. Towards the end of December 2019, a few people working at the market developed a mysterious form of pneumonia. On Dec. 30, public health officials asked hospitals to report any new market-related cases of pneumonia.

It also became clear in late December that a new coronavirus was the cause of the mysterious pneumonia. Coronaviruses have a disturbing history in China: In 2002, another coronavirus sparked the SARS epidemic, which killed 774 people. Scientists later concluded that the virus originated in bats, spread to wild mammals, and then spread to humans in markets where the mammals were sold.

Fearing a repeat of SARS, Chinese authorities ordered the closure of the Huanan market. Wuhan police shut it down on January 1, 2020. Workers dressed in hazmat suits washed and disinfected the stalls.



A member of the Wuhan Hygiene Emergency Response Team inside the closed Huanan Market on January 11, 2020.

Noel Celis/Agence France-Presse

Chinese scientists said they found the virus in dozens of samples taken from surfaces and sewers in the market, but not in swabs taken from animals in the market.

The bond with the market appeared to weaken as the coronavirus spread. Meanwhile, questions have arisen about research being carried out at a lab in the city, the Wuhan Institute of Virology, where scientists studied coronaviruses.

For the new studies, Dr Worobey and colleagues estimated the latitude and longitude of 156 Covid cases in Wuhan in December 2019. The highest case density was concentrated around the market.


Spatial analysis of Covid cases in December 2019.





Huanan Seafood

wholesale market

Concentration of Covid cases in Dec. 2019

Huanan Seafood

wholesale market

Concentration of Covid cases in Dec. 2019


Source: Michael Worobey et al., preprint via Zenodo

The New York Times

The researchers then mapped the cases in January and February. They used data collected by Chinese researchers from Weibo, a social media app that created a channel for people with Covid to seek help. The 737 cases pulled from Weibo were concentrated away from the market, in other parts of central Wuhan with a large population of elderly residents.


Spatial analysis of Covid cases in January-February 2020.





Huanan Seafood

wholesale market

Concentration of Covid cases in Jan.–Feb. 2020

Huanan Seafood

wholesale market

Concentration of Covid cases in Jan.–Feb. 2020


Source: Michael Worobey et al.

The New York Times

Models pointed to the market as the source of the outbreak, studies found, with the coronavirus then spreading to surrounding neighborhoods before moving further into the city. The researchers carried out tests which showed that it was extremely unlikely that such a pattern could be produced simply by chance.

“It’s very strong statistical evidence that it’s not a coincidence,” Dr Worobey said.

The researchers also presented evidence that in late 2019 market vendors were selling raccoon dogs and other mammals known to be potential coronavirus hosts. Genetic samples taken from the floors, walls and other surfaces of Huanan Market in January 2020 reveal traces of SARS-CoV-2 in the southwestern corner of the market, where vendors were clustered.


The Huanan Seafood Wholesale Market in Wuhan, China.





Huanan Seafood

wholesale market

Western coast

Huanan Seafood

wholesale market

Western coast

Huanan Seafood

wholesale market

Western coast



Diagram of the Huanan market.





Huanan Seafood Wholesale Market

Western coast

Coronavirus found in barn

Live mammal sales stand

Stall selling unknown meat

Huanan Seafood Wholesale Market

Western coast

Coronavirus found in barn

Live mammal sales stand

Stall selling unknown meat

Huanan Seafood Wholesale Market

Western coast

Stall where the coronavirus was found

Live mammal sales stand

Stall selling unknown meat



Distribution of coronavirus samples in Huanan market.





Huanan Seafood Wholesale Market

Western coast

Distribution of positive coronavirus samples

on the market

Huanan Seafood Wholesale Market

Western coast

Distribution of positive coronavirus samples

on the market

Huanan Seafood Wholesale Market

Western coast

Distribution of positive coronavirus samples

on the market


Source: Michael Worobey et al.

The New York Times; Satellite image via Google Maps

Further clues emerged when researchers created an evolving family tree of coronaviruses sampled during the early weeks of the pandemic. The tree is divided into two main branches, called A and B. Looking at the mutations in each branch, the researchers concluded that they must have come from animals separately, each adapting to humans on their own.

The B line likely jumped to humans in late November or early December 2019, the researchers believe, while the A line jumped a few weeks after that.

Previous studies had only identified the B lineage in the Huanan market. But Dr Worobey and his colleagues found that the first two lineage A cases were found in people who lived near the market.

The Chinese CDC study released on Friday found lineage A coronavirus on a glove collected during the market close. This finding supports the hypothesis that the two coronaviruses made the leap from animals to market.



Workers in protective suits disinfect the market in Huanan on March 4, 2020.

Reuters

Dr. Bloom, however, questioned the idea that there were two separate spinoffs. He noted that the market lineage A sample was collected some time after the virus started spreading in humans, raising the possibility that it was introduced to the market.

He said the two lineages also only differed by two mutations, and that one could have evolved from the other as the virus passed from person to person.

“I am particularly unconvinced by the conclusion that there must necessarily have been two different fallouts in the Huanan seafood market,” Dr Bloom said.

The new studies do not specify exactly which animals are spreading the coronavirus in the Huanan market. But Dr Worobey and his colleagues said it might be possible to dig deeper into the mystery by analyzing genetic data collected by Chinese researchers. In addition to viral genes, it could also include genes from the mammals that carried them.

This is a developing story.

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