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More Evidence That COVID-19 Started in Wuhan Marketplace

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Editor’s note: Find the latest news and guidance on COVID-19 at the Medscape Coronavirus Resource Center.

Many scientists and public health experts have long said that the coronavirus that caused the global pandemic originated in a market in Wuhan, China. Two new studies support this argument.

Studies have shown that the initial spread of the virus was a double whammy. The virus has been transmitted twice from animals to humans. Virus genetics and outbreak modeling in one study identified two strains released a few weeks apart in November and December 2019.

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“Now I understand that it sounds like I just said that a once-in-a-generation event happened twice in a row, and pandemics are really rare,” said Joel O. Wertheim, Ph.D., at a briefing sponsored by the American Association Development of Science.

For an outbreak to develop into a pandemic, a unique storm of factors had to be present: animals carrying a virus that could be transmitted to humans, close human contact with those animals, and a city large enough for the infection to spread before it could spread. contain examples.

Unfortunately for us humans, this coronavirus – SARS-CoV-2 – is a “generic virus” capable of infecting many animals, including humans.

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“Once all conditions are in place… barriers to proliferation will be lowered,” said Wertheim, a genetic and molecular network researcher at the University of California, San Diego. In fact, in addition to the two strains of the virus that took hold, there were probably up to two dozen other cases where people caught the virus but didn’t spread it everywhere, and it died out.

Overall, the odds were against the virus – in 78% of cases, “introduction” to humans is likely to disappear, the study found.

The study found that the COVID-19 pandemic started small.

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“Our model shows that by early December there were probably only a few dozen infections and only a few hospitalizations due to COVID-19,” said Jonathan Pekar, a graduate student working with Wertheim.

At the end of 2019, Wuhan did not have a single positive coronavirus sample out of thousands of blood samples from healthy donors tested between September and December, Pekar said. Similarly, no blood sample from patients hospitalized with influenza-like illness from October to December 2019 tested positive for SARS-CoV-2.

Outbreak mapping

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A second study published in the journal Science has identified the earliest cases of COVID-19. This effort showed a dense cluster around a seafood wholesale market in Wuhan, a city of 11 million people.

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When the researchers tried other scenarios – simulating outbreaks in other parts of the city – the scheme didn’t work. Again, the Wuhan market proved to be the epicenter of the onset of the pandemic.

Michael Sparrow, Ph.D., and his colleagues used data from Chinese scientists and the World Health Organization for the study.

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“There was this unusual pattern where the highest case density was both very close to this market and very focused on it,” said Sparrow, head of ecology and evolutionary biology at the University of Arizona at Tucson.

The highest density of cases in the city of 8,000 square kilometers was “a very, very small area of ​​about a third of a square kilometer,” he said.

The nature of the outbreak showed that the Wuhan market “is in the middle.”

So, if it started with infected workers at the market, how did it spread from there? The virus likely entered the community when market vendors went to local stores, infecting people in those stores. Then members of the local community not connected to the market began to become infected with the virus, Sparrow said.

Investigators also determined which stalls in the marketplace were most likely involved, a sort of indoor congestion. “This group is very, very specific in those parts of the market where… they were selling wild animals, including, for example, raccoon dogs and other animals that we know are susceptible to infection with SARS-CoV-2,” Christian Andersen said. PhD, Director of Infectious Disease Genomics, Scripps Translational Research Institute, La Jolla, California.

What remains unknown is which animal or animals carried the virus, although the raccoon dog — a fox-like animal found in parts of Asia — remains central to most theories. In addition, many of the farms supplying the animals to the market have since closed, making it difficult for researchers to figure out exactly where the infected animals came from.

“We don’t know for sure, but raccoon dogs were sold in this market until the start of the pandemic,” Andersen said.

Excluding other theories

Sparrow himself was among those who believe that SARS-CoV-2 was released from a laboratory in China. “In the past, I was much more open to the idea of ​​a lab leak,” he said. “And published it in a letter to Science” in November 2021.

The letter had “a lot more impact than I thought, in the sense that I think it turned out to be quite devastating,” he said. As more evidence has since emerged, Sparrow said he came up with the Wuhan market source theory.

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Andersen agreed that he was initially more open to the lab leak theory. “I myself was quite convinced of the lab leak until we dived into it very carefully and looked at it much closer,” he said. The new data convinced him “that in fact the data points to this particular market.”

“Did we debunk the lab leak theory? No,” Anderson said. “Will we ever be able to? Not”. But the scenario of the origin of the Wuhan market is more plausible. “I would say that these two papers together represent the strongest evidence for this to date.”

Andersen said identifying the source of the outbreak that led to the COVID-19 pandemic is based on science. “We are trying to understand the origin of the pandemic. We’re not trying to pin the blame.”

Future Directions

“Because pandemics are pandemics, they affect all of us,” Andersen said. “We cannot prevent the kind of events that led to the COVID-19 pandemic. But we can hope to make sure that outbreaks don’t turn into pandemics.”

Andersen says the future will require rapid data provision and collaboration. Very robust surveillance systems, including wastewater surveillance, can help monitor SARS-CoV-2 and other pathogens of potential future concern.

It should also be standard practice for healthcare professionals to be on the lookout for unusual respiratory infections, the researchers said.

“Damn lucky that the doctors at Shinwa Hospital were so alert that they noticed these cases were something out of the ordinary at the end of December,” Sparrow said. “It shouldn’t have worked that way.”

Sources

Briefing, American Association for the Advancement of Science, July 26, 2022

Science: “The Huanan Seafood Wholesale Market in Wuhan Was an Early Epicenter of the COVID-19 Pandemic”, “Molecular Epidemiology of Multiple Zoonotic Origins of SARS-CoV-2”.

Damian McNamara is a staff journalist based in Miami. It covers a wide range of medical specialties including infectious diseases, gastroenterology, and critical care. Follow Damian on Twitter: @MedReporter.

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