Covid-19 origins: researchers challenge early paper pointing to Wuhan market as epicentre of pandemic
- Mathematicians from Germany and Hong Kong say ‘the market is not more likely to be the origin’ when compared to other nearby landmarks
- Lead author of 2022 study cited in paper says he is ‘working with a colleague on a scientific response’
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”.
The researchers were 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.
“Neither [the 2022 study] nor our statistical analysis could be used to support or reject the zoonosis hypothesis,” they said, referring to an infectious disease that jumps from an animal to humans.
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.
Through mapping, statistical analysis and analysis based on the distribution of the city’s population density, they found that “early cases lived near to and centred on” the Huanan Seafood Wholesale Market in Wuhan.
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.
“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,” the report said.
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.
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 that 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 Prevention and Control, 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.”
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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,” Cowling 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.”
