The Huanan Wholesale Seafood Market in Wuhan, China, was the epicenter of the early outbreaks of SARS-CoV2, a new, peer-reviewed study asserts, following its examination of early COVID-19 cases in the region and environmental samples originating from the market. 

The study, published by Science Magazine, reinforces the theory that the emergence of SARS-COV2 occurred via the live wildlife trade in China, which was an active part of the Huanan market in late 2019 and early 2020. 

These results hone in on preprint versions of the same research, led by the University of Arizona’s Michael Worobey, that piece together spatial and biological evidence about the virus’ presence and evolution among Wuhan residents in early 2020, as well as environmental samples taken from Wuhan’s live animal markets between December 2019 and February 2020.

Higher concentration of cases around the market than expected based on population distirbution and density

The study examined the spatial density and geographical location of 155 of the 174 SARS-CoV2 cases confirmed in Hubei province in December 2019 from the 2021 World Health Organization mission report, for which locations were identified. 

While early COVID-19 cases occurred across Wuhan, the December 2019 cases were all “significantly closer” to the Huanan market than would be expected given Wuhan’s population density and distribution, the researchers found – averaging around 4-6 kilometers from the market in comparision to 16.11 kilometers for the population as a whole.  

The clusters of early cases around the market also occured at a frequency that was far higher than could be expected in comparison to the volumes and frequency of visitors to other major commercial locations in the city.

The researchers also found that both early lineages of SARS-CoV-2, dubbed A and B were “geographically associated” with the market: “Until a report in a recent preprint, only lineage B sequences had been sampled at the Huanan market,” the researchers added.

Finally, the study also found that SARS-CoV2 susceptible mammals, such as red foxes, hog badgers, and common raccoon dogs, were sold at the market in late 2019. Within the market, SARS-CoV2 environmental samples were associated with vendors selling those live mammals. 

“If SARS-CoV2 did not emerge at the Huanan market, how surprised should we be at the coincidence of finding the first cluster of a new respiratory virus at – of all places – one of a handful of markets in a city of 11 million,” said Michael Worobey of the University of Arizona and one of the authors of the study, said in a tweet on the study.  

Majority of early cases clustered near Huanan market 

Interestingly, even ‘unlinked’ early COVID-19 patients, those who neither worked at the market or knew someone who did, nor had not recently visited the market, resided significantly closer to the market than those patients with a direct link.  

“This is clear indication that they were infected as the virus moved from the market into the local community surrounding the market,” said Worobey in a series of tweets about the research.

In fact, these 'unlinked' cases lived significantly closer to and more centered on the market than case epidemiologically linked to the market. This is a clear indication that they were infected as the virus moved from the market into the local community surrounding the market.

— Michael Worobey (@MichaelWorobey) July 26, 2022

This clustering of COVID-19 cases in December contrasts with the pattern of widely dispersed cases across Wuhan by early January through mid-February 2020, which was mapped by data from individuals using a COVID-19 assistance app on the Chinese social media app Weibo. 

Unlike early COVID-19 cases, by January and February many of the sick who sought help resided in a a range of highly populated areas of the city, particularly in areas with a high density of older people.  

Live mammal sales and cases linked to positive SARS-CoV2 samples 

The study also points to the links between live mammal sales and positive environmental samples of SARS-CoV2 found in and around some of the animal stalls after the outbreak began. 

In addition to selling seafood, poultry, and other commodities, the Huanan market was among four markets in Wuhan reported to consistently sell a variety of live, wild-captured or farmed, mammal species in the years and months leading up to the COVID-19 pandemic. 

While there was a general slowdown of live animal sales during winter months, it was reported that animals, including raccoon dogs, were consistently available for sale up until at least November 2019, the study relates.

Five of the SARS-CoV2 positive environment samples were linked to a single stall that sold live mammals in late 2019. Objects sampled showed an association with animal sales, including a metal animal cage, carts used to transport animals, and hair/feather removers. 

This same stall was visited by one of the researchers of the paper in 2014, who had then observed live raccoon dogs housed in a cage at the same stall, on top of a cage with live birds. 

In analyzing potential regions of the market with increased density of positive samples, researchers found that ‘distance to the nearest vendor selling live mammals’ and ‘distance to the nearest human case’ were associated with the positive identification SARS-CoV-2 in environmental samples taken from the stalls. 

Live animal trade and live animal markets ‘common theme’ in virus spillover 

Based on these findings, the researchers described both the live animal trade and live animal markets a “common theme in virus spillover events”. 

“Maximum effort must now be applied to elucidate the upstream events that might have brought SARS-CoV2 into the Huanan market, culminating in the COVID-19 pandemic,” the study concludes.

“To reduce the risk of future pandemics we must understand, and then limit, the routes and opportunities for virus spillover.” 

WHO complements study, but calls for further research 

Commenting on the study at the weekly WHO press briefing, Wednesday, WHO’s technical lead on COVID-19 Dr Maria Van Kerkhove noted that some of the findings presented by Worobey and his colleagues had been noted in the most recent WHO report on the virus origins, published in June 2022 – which drew from pre-print versions of the research.

Overall, the researchers had presented “good analyses” but “without those serologic studies in the markets, at the source farms, without tracing those animals back, it still leaves some open questions,” Van KerKhove said.  

“Unfortunately, it’s not enough. We need more studies to be done in China and elsewhere,” she said, so as to make a definitive determination of the SARS-CoV2 origins. “The goal is not just to figure out how this pandemic began, it’s to learn how we better prepare for the future.”   

Added Dr Mike Ryan, Executive Director of WHO’s Health Emergencies Programme: “It’s important for us to remember that all hypotheses remain on the table. But we’re very pleased to see this kind of work being done, which advances us and advances our common understanding of the origins of this disease.”

Image Credits: Arend Kuester/Flickr, Worobey et al, 26 Science, July 2022, Worobey et al (2022) .