Evidence that the Huanan market in Wuhan, China, was the epicenter of the COVID-19 pandemic

In a recent study published in Science, researchers analyzed data from multiple sources to determine the epicenter of the coronavirus disease 2019 (COVID-19) pandemic.

Background

During the beginning of the COVID-19 pandemic, scientists hypothesized that animals sold at the Huanan market were the source of the inexplicable pneumonia cases in Wuhan, China, which led to its closure on 1 January 2020. A systematic review of all the cases notified to China’s National Notifiable Disease Reporting System showed that 55 of 168 of the earliest known COVID-19 cases were associated with the Huanan market.

Like the Huanan market, there are three other markets in Wuhan where sustained live mammal sales occurred during 2019. However, due to a lack of data regarding upstream events of the market and obscurity of exact circumstances at the market, implicating early cases in the Huanan market does not conclusively establish that the pandemic originated there.

About the study

In the present study, researchers first carefully examined the 2021 World Health Organization (WHO) mission report, which identified 174 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cases in Hubei province in December 2019. They could reliably extract the latitude and longitude coordinates of 155 cases from the maps. Next, the researchers reconstructed probability density contours by a kernel density estimate (KDE) using all 155 COVID-19 cases geo-locations from December 2019 and the 120 COVID-19 cases locations from December 2019 unlinked to the Huanan market. They observed that the location of the Huanan market lay within the highest density 1% contour in both scenarios.

The highest density 50% contour is the area where cases extracted from the probability distribution are as likely to lie inside as outside the Haunan market. The team also identified the highest density 25%, 10%, 5%, and 1% contours. Similarly, they identified 50% to 1%  highest probability density contours for 737 COVID-19 case locations from Weibo data.

Further, the researchers used an empirical null distribution of Wuhan’s population density to determine whether the December COVID-19 cases were closer to the Huanan market. They considered three categories of cases:

(i) all cases,

(ii) cases linked directly to the Huanan market,

(iii) cases unlinked to the Huanan market

Study findings

Most December 2019 SARS-CoV-2 cases in Wuhan clustered in central Wuhan near the west bank of the Yangtze River. The researchers mapped these December 2019 cases using location data from individuals using a COVID-19 assistance app on Sina Weibo. By January and February 2020, COVID-19 cases surged in highly populated areas of Wuhan, particularly where the elderly population resided.

Further, the researchers made two important observations. First, the COVID-19 cases unlinked to the Huanan market on average resided closer to the market than those linked to the market. Second, the distance between the case center points and the Huanan market was shorter than expected for all December 2019 cases relative to the empirical null distribution of Wuhan’s population density.

The center point for all the cases was 1.02 kilometers (km) away. Likewise, the center-point for December 2019 cases linked to the Huanan market and unlinked to the market was 2.28 km and 0.91 km away, respectively. Under the ‘center-point distance to the Huanan market’ test and ‘median distance to Huanan market’ test, the researchers could remove 38 cases residing within a radius of 1.6km and 98 cases living within a radius of 5.8km before losing significance at the α=0.05 level, respectively.

Apart from the Huanan market, no other region in Wuhan showed high case density in a spatial relative risk analysis reported via Weibo. Both SARS-CoV- A and B lineages have co-circulated globally since early in the COVID-19 pandemic. Intriguingly, both these lineages were geographically associated with the Huanan market.

A recent preprint also confirmed the authenticity of the Chinese Center for Disease Prevention and Control (CCDC) report, which documented the early presence of the A lineage of SARS-CoV-2 in a Huanan market environmental sample. Thus, it is likely that SARS-CoV-2 lineage A, like lineage B, might have originated at the Huanan market and then transmitted onwards from this epicenter into the market neighborhoods and then beyond.

Spatial analyses within the market also identified multiple SARS-CoV-2-positive environmental samples, including cages, carts, and freezers, concentrated in the southwest corner of the market, where vendors sold live mammals, including raccoon dogs immediately before the COVID-19 pandemic. These findings suggested that SARS-CoV-2-infected live animals were present at the Huanan market at the beginning of the COVID-19 pandemic.

Conclusions

Despite limited testing of live wildlife sold at the market, the study results evidenced that the Huanan market was the early epicenter of the COVID-19 pandemic, and SARS-CoV-2 likely emerged from the live wildlife trade in China, implying it had a zoonotic origin. Indeed, markets such as the Huanan market selling live mammals pose the highest risk of virus spillover. Concerted efforts are needed to elucidate the upstream events that might have carried SARS-CoV-2 into the Huanan market, terminating in the COVID-19 pandemic to limit the routes and opportunities for virus spillover in the future. Future studies should continue studying the origins of the SARS-CoV-2 pandemic to lower the risk of future pandemics.

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.