A lab leak in Wuhan versus a natural hop from bats: Evidence for each theory of the coronavirus pandemic's origin

• The idea the coronavirus leaked from a lab hasn't been disproved, though a WHO report said it was unlikely.
• The virus probably jumped to people from animals. Biden gave US officials 90 days to assess both ideas.
• Here's a breakdown of the evidence in favor of a lab leak and a natural spillover from animals.

It's the theory that refuses to die: Might the coronavirus have leaked out of a Chinese lab?

As long as the mystery of the pandemic's origin remains unsolved, the question will persist. Increasingly, global leaders are calling for more thorough investigations into the possibility.

That group includes President Joe Biden, Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, and Tedros Adhanom Ghebreyesus, the director-general of the World Health Organization.

Though a month-long WHO investigation in the city of Wuhan concluded that the coronavirus most likely spilled over to people from animals - possibly at wildlife farms - the group found no definitive proof of that. Nor could it rule out a lab leak.

So Tedros said in March that he did "not believe that this assessment was extensive enough."

Fauci, meanwhile, said during a Senate hearing this month that the "possibility certainly exists" that the pandemic started because of a lab accident. Scott Gottlieb, the former head of the Food and Drug Administration, has also said there is some circumstantial evidence favoring a lab leak, as has Robert Redfield, the former director of the Centers for Disease Control and Prevention.

In a Wednesday press conference, Biden gave the US intelligence community 90 days to collect and analyze evidence supporting each of the two scenarios, in the hopes of reaching a "definitive conclusion" on the coronavirus' origin.

Here's what to know about each theory - a lab leak and a natural spillover from animals - and the key pieces of evidence supporting each.

The lab-leak hypothesis

Eighteen scientists from the US, the UK, Canada, and Switzerland recently published a letter saying they thought the lab-leak theory remained viable.

Questions about a such a leak generally center on the Wuhan Institute of Virology, a high-level biosafety lab where some scientists had been studying coronaviruses before the pandemic. Wuhan, of course, is the city where authorities reported the first known cluster of COVID-19 cases. Below are the main reasons people think the virus might have emerged from the lab.

The Wuhan Institute of Virology was researching coronaviruses before the pandemic

Scientists at the WIV research infectious diseases - collecting, storing, and genetically analyzing samples of the most dangerous and infectious pathogens known to humankind. The institute boasts a biosafety level 4 lab, one of only a few dozen in the world.

Peter Ben Embarek, a WHO scientist specializing in animal disease, was part of the team that investigated the institute in January. He said it's natural to speculate about a link - especially given that the WIV moved to a new location in early December 2019, which happens to be just miles from the Huanan Seafood Market.

The first cluster of coronavirus cases in Wuhan was linked to the market, but it turned out to simply have hosted an early superspreader event.

"Even the staff in these labs told us that was their first reaction when they heard about this new emerging disease, this coronavirus: 'This is something coming out of our labs,'" Ben Embarek said in March.

But after investigating that possibility, the WIV staff said they found no evidence that samples of the new coronavirus had been stored at the institute prior to December 2019. Records reviewed by WHO did not indicate that any viruses closely related to the new coronavirus were kept in any Chinese lab before that month. The records also did not show any viruses that, when combined, could have produced the new coronavirus.

But Ben Embarek's team also said it wasn't given full access to the Wuhan institute's data.

WHO investigators couldn't conduct a full audit of the labs

Ben Embarek said he and his fellow investigators didn't do a full audit of the WIV. The WHO team spent just hours at the institute - which isn't enough time to pore over files, databases, or freezer inventories. The institute's staff also did not share all of its records or safety logs.

That's why Tedros, Fauci, and many others are still calling for a full investigation of the lab.

However, Jonna Mazet, an epidemiologist at the University of California at Davis, has worked directly with WIV researchers, including one of its prominent virologists, Shi Zhengli. Mazet told Insider the lab's records were above reproach.

"She is absolutely positive that she had never identified this virus prior to the outbreak happening," Mazet told Insider, referring to Shi's work.

WIV staff members got sick with 'COVID-like' symptoms in November 2019

A report uncovered by The Wall Street Journal revealed that three WIV staff members got sick and went to a hospital more than a month before experts identified the first COVID-19 cases in Wuhan. The report - which an intelligence official said lacked sufficient corroboration - said the workers' symptoms were "consistent with both COVID-19 and common seasonal illness."

According to the virologist Marion Koopmans, WHO team was aware that some WIV staff had gotten sick in the fall of 2019. They'd chalked the incidents up to season illness because blood samples taken from WIV staff in the months ahead of the pandemic all tested negative for coronavirus antibodies. (Such samples are taken routinely from biosafety lab workers to monitor their health.)

The coronavirus is easily transmissible among humans

Generally, it takes time for a new virus to adapt to be able to spread easily from person to person.

So people like Redfield point to the coronavirus' highly infectious nature as evidence it may be a product of "gain-of-function" research. In this kind of work, scientists tweak viruses with the goal of making the pathogens more transmissible or deadlier to figure out how to stop future pandemics.

"I do not believe that this somehow came from a bat to a human, and at that moment in time that the virus came to the human became one of the most infectious viruses that we know in humanity for human-to-human transmission," Redfield told CNN in March.

But Fauci said that same month that it's more likely that the coronavirus got good at jumping between people while spreading "below the radar" in China in late 2019. Growing evidence suggests COVID-19 was spreading for several weeks, if not months, before the first cases were reported.

That allowed the virus "to be pretty well adapted when first recognized," Fauci said.

Lab leaks happen, and US intelligence suggested the WIV had poor safety protocol

Three years ago, US officials visiting Wuhan sent a pair of memos to the State Department warning of inadequate safety measures at the lab. The institute seems to have made rigorous changes since then, though, and the WHO team was satisfied with the lab's protocols.

Ben Embarek said the WIV housed a "state-of-the-art lab," which is part of the reason his team thinks it's "very unlikely that anything could escape from such a place."

Mazet, too, has said it's "highly unlikely this was a lab accident," since she worked with WIV staff to develop and implement a "very stringent safety protocol."

Still, Ben Embarek noted in February that "accidents do happen."

"We have many examples in many countries in the world of past accidents," he said.

Though such accidents are rare, there have been four instances in which SARS has leaked from laboratories in Taiwan, Singapore, and Beijing.

The wildlife farms where the virus might have emerged are 1,000 miles from Wuhan

The wildlife farms where the WHO team thinks the coronavirus most likely emerged are 800 to 1,000 miles from Wuhan.

But Koopmans said the WHO team found that rabbits and ferret-badgers sold at Huanan Seafood Market were transported there from regions in China where bats harbor viruses similar to the new coronavirus. Both rabbits and ferret-badgers are susceptible to coronavirus infection, so could have passed it to farmers who traveled into the city, or to market shoppers.

Still, just because the first reported cluster of cases emerged in Wuhan doesn't mean that's where the pandemic truly began. Wuhan is the largest city in Hubei province, and people from all over central China travel though the region. Once the virus arrived in a dense, urban environment, it makes sense it would spread rapidly there.

The animal-spillover theory

After the investigation in Wuhan, the WHO team determined the coronavirus "most likely" jumped from bats to people via an intermediary animal host at a wildlife farm. This kind of spillover has been the leading theory throughout the pandemic primarily because 75% of new infectious diseases come to us from animals. Plus, the coronavirus' genetic code is very similar to that of other coronaviruses found circulating in bats. Here's the evidence supporting this idea.

The WHO concluded that an animal-to-human hop is 'most likely'

In southern China, the WHO found, people interacted closely with animals like civets, minks, pangolins, rabbits, and raccoon dogs at farms where these animals were bred in captivity for food.

All of these species can be infected by the new coronavirus, and any contact with an infected animal or its poop can allow a virus to jump from animals to people. That's why the WHO found this to be the "most likely" origin of the pandemic. Still, the team examined 80,000 animals from 31 provinces across China and didn't find a single case of the coronavirus. China shut down the specific wildlife farms in question in February 2020, and the WHO researchers weren't given access to samples from animals from these farms.

Plus, according to Tedros, the WHO experts had difficulties accessing COVID-19 infection data and patient blood samples from in and around Wuhan - which could also cast doubt on the team's conclusions.

The scientists behind the recent letter about the lab-leak theory wrote that in the WHO's report, that possibility was "not given balanced consideration." Only four of the report's 313 pages discuss evidence of a lab accident.

SARS-CoV-2 shares 97% of its genetic code with other coronaviruses found in bats

Bats are common virus reservoirs. Cross-species hops from bat populations also led to the outbreaks of Ebola, SARS, and the Nipah virus.

A wealth of evidence shows similarities between the new coronavirus and coronaviruses in bat populations. A May 2020 study, for example, revealed that the new coronavirus shared 97.1% of its genetic code with a coronavirus called RmYN02, which was found in bats in China's Yunnan province between May and October 2019. A paper in the journal Nature, published by Shi's group at the WIV, found that a coronavirus named RaTG13 was a 96.2% match.

RaTG13, it turns out, is the same virus that Shi and her WIV colleagues collected samples of nearly a decade ago in a remote mine. Six miners got a mysterious pneumonia-like illness there in 2012, and three of them died, according to The Wall Street Journal. Blood samples from the miners didn't test positive for the new coronavirus, however.

When Shi and coauthors published their genetic analysis of RaTG13 last year, they did not disclose its link to the miners' deaths.

Three-quarters of infectious diseases come from natural spillover

Three out of every four emerging infectious diseases come to us from other species; these pathogens are known as zoonotic diseases.

Peter Daszak, a disease ecologist with EcoHealth Alliance who was a member of the WHO investigation team, told NPR in April 2020 that "1 to 7 million people" were exposed to zoonotic viruses in Southeast Asia each year.

"That's the pathway," he said. "It's just so obvious to all of us working in the field."

Daszak and the EcoHealth Alliance have worked with and funded WIV research in the past, though that funding was canceled last year. Some people suggest Daszek has a bias against the lab-leak theory, since it could lead his organization to be seen as culpable for funding research that led to the pandemic.

Still, spillover events have doubled - if not tripled - in the past 40 years, according to Dennis Carroll, the former director of USAID's emerging-threats division. That's because people are increasingly turning wild areas into farms and fields for livestock production.

"Whatever future threats we're going to face already exist - they are currently circulating in wildlife," Carroll told Nautilus Magazine last year.