What you need to know about the COVID-19 lab-leak hypothesis

Newly reported information has revived scrutiny of this possible origin for the coronavirus, which experts still call unlikely though worth investigating.

Months after a World Health Organization investigation deemed it “extremely unlikely” that the novel coronavirus escaped accidentally from a laboratory in Wuhan, China, the idea is back in the news, giving new momentum to a hypothesis that many scientists believe is unlikely, and some have dismissed as a conspiracy theory.

The renewed attention comes on the heels of President Joe Biden’s ordering U.S. intelligence agencies on May 26 to “redouble their efforts” to investigate the origins of the coronavirus. On May 11, Biden’s chief medical adviser, Anthony Fauci, acknowledged he’s now “not convinced” the virus developed naturally—an apparent pivot from what he told National Geographic in an interview last year. 

Also last month, more than a dozen scientists—top epidemiologists, immunologists, and biologists—wrote a letter published in the journal Science calling for a thorough investigation into two viable origin stories: natural spillover from animal to human, or an accident in which a wild laboratory sample containing SARS-CoV-2 was accidentally released. They urged that both hypotheses “be taken seriously until we have sufficient data,” writing that a proper investigation would be “transparent, objective, data-driven, inclusive of broad expertise, subject to independent oversight,” with conflicts of interest minimized, if possible.

“Anytime there is an infectious disease outbreak it is important to investigate its origin,” says Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins University Center for Health Security who did not contribute to the letter in Science. “The lab-leak hypothesis is possible—as is an animal spillover,” he says, “and I think that a thorough, independent investigation of its origins should be conducted.”

Unanswered questions

The origins of SARS-CoV-2, the virus that causes COVID-19 and has infected more than 171 million people, killing close to 3.7 million worldwide as of June 4, remain unclear. Many scientists, including those that participated in the WHO’s months-long investigation, believe the most likely explanation is that that it jumped from an animal to a person—potentially from a bat directly to a human, or through an intermediate host. Animal-to-human transmission is a common route for many viruses; at least two other coronaviruses, SARS and MERS, were spread through such zoonotic spillover.

Other scientists insist it’s worth investigating whether SARS-CoV-2 escaped from the Wuhan Institute of Virology, a laboratory that has studied coronaviruses in bats for more than a decade.

The WHO investigation—a joint effort between WHO-appointed scientists and Chinese officials—concluded it was “extremely unlikely” the highly transmissible virus escaped from a laboratory. But the WHO team suffered roadblocks that led some to question its conclusions; the scientists were not permitted to conduct an independent investigation and were denied access to any raw data. (We still don’t know the origins of the coronavirus. Here are 4 scenarios.)

On March 30, when the WHO released its report, its director-general, Tedros Adhanom Ghebreyesus, called for further studies. “All hypotheses remain on the table,” he said at the time.

Then on May 11, Fauci told PolitiFact that while the virus most likely emerged via animal-to-human transmission, “it could have been something else, and we need to find that out.”

Recently disclosed evidence, first reported by the Wall Street Journal, has added fuel to the fire: Three researchers from the Wuhan Institute of Virology fell sick in November 2019 and sought hospital care, according to a U.S. intelligence report. In the final days of the Trump administration, the State Department released a statement that researchers at the institute had become ill with “symptoms consistent with both COVID-19 and common seasonal illness.”

Most epidemiologists and virologists who have studied the novel coronavirus believe that it began spreading in November 2019. China says the first confirmed case was on December 8, 2019. During a briefing in Beijing this week, China’s foreign ministry spokesperson, Zhao Lijian, accused the U.S. of “hyping up the theory of a lab leak,” and asked, “does it really care about the study of origin tracing, or is it trying to divert attention?” Zhao also denied the Wall Street Journal report that three people had gotten sick.

Lab leak still ‘unlikely’

Some conservative politicians and commentators have embraced the lab-leak theory, while liberals more readily rejected it, especially early in the pandemic. The speculation has also heightened ongoing tensions between the U.S. and China.

On May 26, as the U.S. Senate passed a bill to declassify intelligence related to potential links between the Wuhan laboratory and COVID-19, Missouri Senator Josh Hawley, a Republican who sponsored the bill, said, “the world needs to know if this pandemic was the product of negligence at the Wuhan lab,” and lamented that “for over a year, anyone asking questions about the Wuhan Institute of Virology has been branded as a conspiracy theorist.”

Peter Navarro, Donald Trump’s former trade adviser, asserted in April 2020 that SARS-CoV-2 could have been engineered as a bioweapon, without citing any evidence.

The theory that SARS-CoV-2 was created as a bioweapon is “completely unlikely,” says William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center. For one thing, he explains, for a bioweapon to be successful, it must target an adversarial population without affecting one’s own. In contrast, SARS-CoV-2 “cannot be controlled,” he says. “It will spread, including back on your own population,” making it an extremely “counterproductive biowarfare agent.”

The more plausible lab-leak hypothesis, scientists say, is that the Wuhan laboratory isolated the novel coronavirus from an animal and was studying it when it accidentally escaped. “Not knowing the extent of its virulence and transmissibility, a lack of protective measures [could have] resulted in laboratory workers becoming infected,” initiating the transmission chain that ultimately resulted in the pandemic, says Rossi Hassad, an epidemiologist at Mercy College.

But Hassad adds he believes that this lab-leak theory is on the “extreme low end” of possibilities, and it “will quite likely remain only theoretical following any proper scientific investigation,” he says.

Biden ordered U.S. intelligence agencies to report back with their findings in 90 days, which would be August 26.

Based on the available information, Eyal Oren, an epidemiologist at San Diego State University, says it’s apparent why the most accepted hypothesis is that this virus originated in an animal and jumped to a human: “What is clear is that the genetic sequence of the COVID-19 virus is similar to other coronaviruses found in bats,” he says.

Some scientists remain skeptical that concrete conclusions can be drawn. “At the end, I anticipate that the question” of SARS-CoV-2’s origins “will remain unresolved,” Schaffner says.

In the meantime, science “moves much more slowly than the media and news cycles,” Oren says.

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