WHO and Chinese scientists concluded it's 'extremely unlikely' the coronavirus leaked from a lab, after a 4-week investigation

• Experts from the WHO and China conducted a month-long investigation into the coronavirus' origins.
• They concluded that the novel virus almost certainly did not leak from a Chinese lab.
• The virus most likely jumped from bats to an intermediary animal host and then to humans.

A year into the pandemic, the origins of the coronavirus are still a mystery.

The World Health Organization sent an international team to Wuhan, China, in January to investigate where the virus came from and how it was introduced into the human population. Yet the investigation yielded few definitive answers.

The experts were able to take one hypothesis off the table, however: the idea that the novel coronavirus accidentally leaked from a Chinese lab. This unsubstantiated theory was pushed by some members of the Trump administration in the spring. But Peter Ben Embarek, a WHO scientist specializing in food safety and animal disease, said in a Tuesday press conference that a lab leak was "extremely unlikely to explain the introduction of the virus into the human population."

The WHO team will not be revisiting that hypothesis in future studies, he said, adding that the virus most likely jumped from an animal host into people - as the Ebola and SARS viruses did.

'Very unlikely that anything could escape from such a place'

Working together with Chinese scientists, the WHO experts had unfettered access to key places in Wuhan over four weeks, including hospitals, laboratories, and the Huanan Seafood Market, which was linked to the first known cluster of COVID-19 cases.

People who've suggested the possibility of a lab leak point mostly to the Wuhan Institute of Virology, since some scientists there study coronaviruses. It's one of the highest-level biosafety labs in the world. There's no evidence, however, that any coronavirus sample collected for study in that lab was accidentally released.

The WHO team spoke with managers and staff at the institute and other labs in the area about their safety protocols.

"Accidents do happen," Ben Embarek said. "We have many examples in many countries in the world of past accidents." But he added that such accidents were rare, that the Wuhan institute housed a "state-of-the-art lab," and that it was "very unlikely that anything could escape from such a place."

Jonna Mazet, a US epidemiologist who has worked with and trained researchers at the institute, also previously told Insider that "it's highly unlikely this was a lab accident."

Mazet said she helped the staff develop and implement a "very stringent safety protocol."

In fact, the WHO team found no evidence that samples of the novel coronavirus existed in any Wuhan labs, or any other labs in the world, before the pandemic.

Shi Zhengli, a virologist at the Wuhan institute, told Scientific American in April that she personally checked and found that none of the coronavirus samples stored at the institute matched the novel coronavirus' genome.

"That really took a load off my mind," Shi said. "I had not slept a wink for days."

The virus most likely jumped from bats to an intermediary host and then to people

Most experts agree that the novel coronavirus, SARS-CoV-2, jumped from an animal host to humans in what's known as a spillover event.

Bats are common virus hosts: Cross-species hops from bats also led to outbreaks of Ebola and SARS. So labs around the world have analyzed genetic samples of bat coronaviruses that were circulating before the pandemic. One study found last February that the new coronavirus shared 96% of its genetic code with a coronavirus seen in Chinese bat populations. Then a May study revealed a closer match, a 97.1% similarity to a coronavirus called RmYN02, which was found in bats in China's Yunnan province between May and October 2019.

According to the WHO team, it's unlikely the virus hopped directly from bats to people, though.

"Our initial findings suggest that the introduction through an intermediary host species is the most likely pathway and one that will require more studies and more specific targeted research," Ben Embarek said.

Animals that might have served as that intermediary host include pangolins, ferrets, minks, snakes, or turtles.

Marion Koopmans, a WHO virologist, said in the press conference that the team found that rabbits and ferret-badgers sold at the Huanan Seafood Market came from regions in China where bats and other animals harbor viruses similar to the new coronavirus. Both of those species are susceptible to SARS-CoV-2 so could have been intermediary hosts.

But the WHO experts couldn't confirm how the new coronavirus arrived at the Huanan market. It's possible that an infected person passed the virus to others at the market, or "it could also be through the introduction of a product," Ben Embarek said.

"A frozen wild animal that was infected could be a potential vehicle of the virus into market environments," he added.

Still, the cases linked to the Huanan market were not the first in China.

The virus wasn't engineered in a lab, either

A conspiracy theory similar to the lab-leak idea is that the new coronavirus was engineered in a Wuhan laboratory. A group of Chinese virologists with ties to the former Trump strategist Steve Bannon suggested in September that Chinese scientists engineered the virus using existing bat coronaviruses as a "backbone" or "template."

One of those virologists, Li-Meng Yan, told the Fox News host Tucker Carlson that the virus was "man-made" and "intentionally" released by the Chinese government.

But that theory "has already been refuted by the scientific community around the world," Liang Wannian, a member of China's National Health Commission who assisted with the WHO investigation, said at the press conference.

A March study published in the journal Nature concluded that based on genetic analysis the new coronavirus wasn't a hodgepodge of existing coronaviruses. The authors found it could not be a "laboratory construct" or "purposefully manipulated virus."

"The genetic data irrefutably show that SARS-CoV-2 is not derived from any previously used virus backbone," they said.