Covid-19 origins: researchers challenge early paper pointing to Wuhan market as epicentre of pandemic
Researchers have analysed early Covid-19 cases and challenged a conclusion about how the pandemic started.
A previous study concluded that a market in the central Chinese city of Wuhan was the early epicentre of the Covid-19 pandemic but a pair of statisticians in Germany and Hong Kong said the “the statistical conclusion is invalid”.
Dietrich Stoyan, a mathematician at TU Bergakademie Freiberg in Germany, and Chiu Sung-nok, a professor in the department of mathematics at Hong Kong Baptist University, said the analysis published in 2022 “does not give an acceptable argument for the centrality of the market in the 155 December cases,” referring to early coronavirus patients in late 2019 whose home addresses were used to show that the cases were geographically centred around the market.
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Despite their criticism of data analysis methods, the statisticians did not offer theories about how or where the pandemic started. They said “neither [the 2022 study] nor our statistical analysis could be used to support or reject the zoonosis hypothesis”, referring to an infectious disease that jumps from an animal to human.
The paper was published in the peer-reviewed Journal of the Royal Statistical Society Series A: Statistics in Society on January 16. It was first uploaded to the preprint server arXiv in August 2022 ahead of peer review, one month after the 2022 study – described by Stoyan and Chiu as a “prominent study” – was published.
In the 2022 study published in the peer-reviewed journal Science, a global team of scientists said their results “provide evidence that the Huanan market was the early epicentre of the Covid-19 pandemic and suggest that Sars-CoV-2 likely emerged from the live wildlife trade in China”.
They found that “early cases lived near to and centred on the Huanan market” through mapping, statistical analysis and analysis based on the distribution of the city’s population density.
They also found that early patients who did not have links with the market “resided significantly closer to the market than those who worked there, indicating that they had been exposed to the virus at or near the Huanan market”.
Lead author of the 2022 study Michael Worobey said he was “working with a colleague on a scientific response” to the paper written by Chiu and his then-PhD supervisor Stoyan.
“Stoyan and Chiu’s study is riddled with errors, both factual and statistical,” said Worobey, a professor and department head of ecology and evolutionary biology at the University of Arizona.
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Scientists have been trying to find out its precise role in the outbreak, as the source of the initial transmission of the virus or as an amplifier of the outbreak. Understanding how the coronavirus emerged is considered key to preventing the next pandemic.
In March 2021, a World Health Organization-China joint mission investigating the origin of Covid-19 said “no firm conclusion … about the role of the Huanan market in the origin of the outbreak, or how the infection was introduced into the market, can currently be drawn” after fieldwork in Wuhan which included visits to the Huanan market and another wholesale market.
“Many of the early cases were associated with the Huanan market, but a similar number of cases were associated with other markets, and some were not associated with any markets,” according to their report.
The joint mission’s report said transmission in the wider community in December 2019, along with the presence of early cases unrelated to the Huanan market, could suggest the market was not the original source of the outbreak.
The Huanan seafood wholesale market in Wuhan, China. Photo: Kyodo
In their recent paper, Stoyan and Chiu said “the assumption that a centroid [centre point] of early case locations or another simply constructed point is the origin of an epidemic is unproved” and that a statistical test “used to conclude that no other location than the seafood market can be the origin is flawed”.
Chiu said that even if the seafood market could be established as the centre of the cases, it did not imply causality.
“We do not agree that the centre location of early cases entails where the explosive outbreak started because the source of the infectious disease could be moving,” he said.
Chiu and Stoyan mapped out the residential addresses of 155 early coronavirus cases used in the 2022 study and found that other landmarks could also be at their centre.
“In the context of statistics, the Wanda Plaza may be more suspicious than the market, which is neither more nor less likely to be the origin than the other landmarks,” they wrote.
They said in terms of statistics “the market is not more likely to be the origin” when compared to other nearby landmarks.
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“[The 2022 study] excluded all landmarks because they claimed that ‘no other location except the Huanan market [is] clearly epidemiologically linked to early Covid-19 cases’,” Chiu and Stoyan wrote.
“In other words, according to [their] approach, if epidemiological links can be found between the cases and any of these landmarks, then these landmarks will be equally likely to be the origin of the pandemic,” they wrote.
They said possible alternative centres could include the Wuhan Centre for Disease Control and Prevention, the Hankou railway station and the Wanda Plaza shopping complex, noting that they “do not hypothesise that any of these landmarks is the origin of the pandemic”.
“We do not aim to refute any hypothesis, whether zoonosis or lab leak. While we pointed out the statistical analysis in the previous study was unconvincing, we did not prove or disprove its conclusion,” Chiu said.
“From the point of view of a statistician, the origin of Covid-19 remains an unanswered question,” he said.
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Ben Cowling, an epidemiologist and medical statistician at the School of Public Health at the University of Hong Kong and who was not involved in either study, said he found Stoyan and Chiu’s argument “compelling”. He said they “do not provide any inference on where the pandemic emerged”.
“I think it would be very difficult now to find any new data relating to the origins of Covid-19, but as explained by Stoyan and Chiu we cannot draw strong inferences from the data that are available,” the chair professor of epidemiology said.
Cowling said any analysis of the early cases, which were all severe, might not capture the full picture of the early epidemic.
“Only about 5 per cent of Covid-19 cases were severe at that time, so for every case that was identified probably there were at least another 19 cases that were not identified. In addition, not every severe case was identified.”
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He outlined the two main hypotheses about the role of the Huanan market.
“There is a hypothesis that the earliest human infections occurred here (in October or November [2019]), perhaps in people who worked in the market or frequently visited the market, and that is why the earliest recorded cases (in December) were clustered around the market,” Cowling said.
“A separate hypothesis is that the earliest human infections occurred elsewhere, but there were just a small number of cases in the early weeks of the epidemic until one of the cases went to the Huanan market and caused a superspreading event in the market, and that is why so many of the detected early cases were linked to the market.”
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