A view of the still-closed Huanan Seafood Wholesale Market in Wuhan.

 Hector Retamal, Agence France-Presse

A University of Arizona scientist was part of an international research team that has concluded the COVID-19 virus originated with live animals sold at the Huanan Seafood Wholesale Market in Wuhan, China.

Michael Worobey, who is a virus evolution expert at the UA, co-authored two peer-reviewed scientific papers published Tuesday in the journal Science that explain these findings.

The first paper focuses on the geographic patterns of the virus in its earliest known period of human transmission: December 2019. Using data gathered by Chinese researchers for the World Health Organization, the authors found that nearly all of the 174 cases (155) from that time originated in Wuhan, and specifically near the Huanan Market.

According to Worobey’s and his colleagues’ findings, many of those early COVID-19 patients had no known interactions with the market but lived nearby.

That’s consistent with the idea that the virus originated in that specific market. Vendors got infected at the market, and as they brought goods to the surrounding community, it spread.

“In this 8,000-square-kilometer city, there was this very small area — about a one-third of a kilometer square — smack dab in the middle,” Worobey said at a virtual news conference Tuesday. “If you understand that, you understand that there’s a very, very strong case that the Huanan market was the epicenter.”

Later cases, however, were more concentrated around the highest population centers in the city of 11 million people. That change in trend suggests that by that point, the highly transmissible virus had spread from its epicenter at the market to the most densely populated areas across the city.

“You really need a big city for a virus like this to take hold and become a pandemic,” said Worobey, who explained that an area with a small population wouldn’t have been conducive to the COVID-19 virus’ rapid spread. “This is a virus you should expect to emerge in a big city, not a rural place.”

A second paper published Tuesday found that the virus’s spread from animals to humans likely occurred in two separate instances, from two separate viral lineages, in late November 2019.

Both of those independent events are believed to have happened at the Huanan market, possibly originating from two different species of animals sold there.

What about the lab-leak theory?

To date, the COVID-19 virus has killed 6.4 million people across the globe, including more than 1 million Americans and 29,000 Arizonans.

In the nearly three years since the pandemic took hold, pundits, politicians and social media personalities have offered their speculation about where the virus originated.

One popular, but unsubstantiated, theory is that the pandemic happened after the virus leaked from a Chinese lab.

In May 2021, Worobey and 17 other scientists published a letter, also in the journal Science, calling on the scientific community to “take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data.”

More than a year later, Worobey is more confident than ever that the so-called lab leak theory is a dead end.

“At that point, we didn’t have a lot of the evidence we do now,” Worobey said Tuesday. “It has moved me to the point where I also think it’s just not plausible that this virus was introduced any other way than through the wildlife trade at the Huanan market.”

His co-author, Kristian Andersen, a professor at Scripps Research in San Diego, agreed.

“Have we disproven the lab leak theory? No, we have not. Will we ever be able to? No,” he said. Andersen added that formal, definitive proofs as they exist in mathematics rarely exist in biology.

But Andersen also cleared up another misconception about what scientists do know about the COVID-19 virus.

“There is this general sense that there’s no data whatsoever that can tell us anything about the origins of the COVID-19 pandemic, which is just untrue,” Andersen said. “These two papers in particular say we do have a lot of evidence — importantly, unfiltered early evidence — that tells us a lot about the origin of this particular pandemic.

“All of that evidence tells us the same thing. It points right to the market in the middle of Wuhan.”

Moving forward, Worobey, Andersen and the many other scientists who are working to further understand the origins and behavior of this pandemic still have unanswered questions.

But to find those answers, Andersen said, the global community will need to stay focused on its end goal.

“Importantly, all of this is only possible if we focus on the collaborative aspect of this,” Andersen said. “If we are trying to place blame for a pandemic, none of this work is ever going to happen.”

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Kathryn Palmer covers higher education for the Arizona Daily Star. Contact her at kpalmer@tucson.com or 520-496-9010.