TUCSON, Ariz. — The origins of COVID-19 have been a mystery since the virus emerged in 2019. Scientists have traced the virus — which has killed over six million globally — to the Chinese city of Wuhan. However, what’s still unclear is how COVID became a deadly pandemic. Did the virus naturally jump from animals to people, or did local researchers create it in a lab? Now, two new studies may have found an answer.
An international team says live animals sold at the Huanan Seafood Wholesale Market in Wuhan were the likely source of the pandemic and that the virus jumped from animals to humans on more than one occasion. The studies also claim that alternative COVID-19 origin theories are extremely unlikely.
In the first study, researchers from the University of Arizona and Scripps Research analyzed the geographic pattern of COVID-19 cases in the first month of the outbreak in December 2019. Researchers were able to determine the locations of nearly all of the 174 coronavirus cases identified by the World Health Organization at that time. Of those 174 cases, 155 were in Wuhan.
These cases were tightly congregated around the Huanan market, with later cases spreading widely throughout Wuhan — a city of about 11 million people. Researchers discovered that a large percentage of early COVID-19 patients had no history of recently visiting the Wuhan market but resided significantly closer to it than other people.
Lead author Michael Worobey, an evolution expert at the University of Arizona, says this shows that the market was the epicenter of the pandemic — with vendors getting sick first and then setting off a chain of infections among the community.
“In a city covering more than 3,000 square miles, the area with the highest probability of containing the home of someone who had one of the earliest COVID-19 cases in the world was an area of a few city blocks, with the Huanan market smack dab inside it,” says Worobey, who heads the university’s Department of Ecology and Evolutionary Biology, in a university release.
Worobey says another finding was able to support this conclusion. When researchers looked at the geographical distribution of later COVID cases, from January and February 2020, they found a “polar opposite” pattern. Those cases coincided with areas of the highest population density in Wuhan, while the cases from December 2019 mapped “like a bullseye” on the market.
“This tells us the virus was not circulating cryptically,” notes Worobey. “It really originated at that market and spread out from there.”
Which animals spread the virus?
In the first study, researchers also examined swab samples from market surfaces like floors and cages after the Huanan market closed. Samples testing positive for the SARS-CoV-2 virus tended to come from stalls selling live wildlife.
Researchers determined that animals now known to be susceptible to the virus, including red foxes, hog badgers, and raccoon dogs, were sold in the Huanan market in the weeks prior to the first recorded COVID-19 cases. Researchers ended up developing a detailed map of the market and showed that positive samples reported by Chinese scientists in early 2020 had a clear association with the western portion of the market. This area is where merchants butchered meat or sold live animals in late 2019.
“Upstream events are still obscure, but our analyses of available evidence clearly suggest that the pandemic arose from initial human infections from animals for sale at the Huanan Seafood Wholesale Market in late November 2019,” explains Kristian Andersen, co-senior author of both studies and a professor in the Department of Immunology and Microbiology at Scripps Research.
Animal transmission was likely rampant in Wuhan
For the second study, researchers combined epidemic modeling with analyses of the virus’ early evolution based on the earliest sampled genomes. Scientists determined the pandemic likely arose from at least two separate infections of humans due to animal contact at the Huanan market in November and possibly December 2019. Their analyses also suggested there were several other animal-to-human transmissions of the virus at the market, but these failed to develop into COVID-19 cases.
Researchers conclude that COVID-19 originated through jumps from animals to humans at the Huanan market. These animals likely contracted the virus from coronavirus-carrying bats in the wild or on local farms in China.
“To more fully understand the origin of SARS-CoV-2, we need to more fully understand events upstream of the Huanan market, which will require close international collaboration and cooperation,” Andersen concludes.
The findings are published in the journal Science.