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The Case Against the Covid-19 Lab Leak Theory

Most theories about the pandemic starting with a bioengineered virus are less plausible than the simpler alternative: bats being bats.

Two researchers in protective gear work in a lab.
Feature China/Barcroft Media/Getty
A virologist works with her colleague in the P4 lab of the Wuhan Institute of Virology, in Wuhan, in central China’s Hubei province, in 2017.

In the past month, we’ve seen a second wave of interest in the theory that the Covid-19 pandemic began with a lab leak. Last spring, the media accurately reported the scientific consensus that Covid (also known as SARS CoV-2) is a natural virus that probably evolved much the same way as the last two deadly human coronaviruses, Severe Acute Respiratory Syndrome (SARS-COV) and Middle East Respiratory Syndrome (MERS-COV): namely, from bats, probably by way of an intermediate species like a raccoon dog, a ferret badger, or even a feral cat. Ever since the SARS outbreak of 2002–03, after all, paper after paper and countless popular pieces have warned that, sooner or later, nature would produce the next big SARS.

Now some—latching on to new reporting, researchers’ open letter to Science magazine calling for greater investigation into the virus’s origins, and the Biden administration’s willingness to use the possibility of a lab leak to demand more transparency from China—argue media outlets were too quick to dismiss the lab leak theory. Substacker Matt Yglesias even called the media’s deeply skeptical coverage of lab leak theories a “fiasco” and “a huge fuckup.”

Despite the allure of this contrarianism, though, 20 years of post-SARS research into the origins and spread of bat coronaviruses point to a natural origin for Covid-19. Upon closer inspection, the so-called “new” evidence that has entranced pundits is neither new nor compelling. Lab leak theory proponents are also glossing over serious flaws in their proposed narratives of Covid-19’s origin. And loose talk about a lab leak elevates tensions between China and the United States, undermining the collaborative research we need to understand this pandemic and prevent the next one.

There are two major variants of the “lab leak” theory in circulation: (1) Covid evolved naturally and leaked pure and uncut from the Wuhan Institute of Virology when researchers were archiving wild bat coronaviruses for study, and (2) scientists at the WIV engineered or altered Covid from a natural virus or viruses, either through benevolent gain-of-function research, in which scientists make a virus more dangerous in order to understand it better, or through bioweapons research. (Republican Senator Tom Cotton of Arkansas entertained a particularly fringe version of this last variation, speculating that China might even have attacked Wuhan with a biological weapon.)

On May 23, The Wall Street Journal reported that three employees at the Wuhan Institute of Virology sought hospital care for respiratory symptoms in mid-November. This could have been an early cluster of Covid cases, even though the symptoms were also consistent with seasonal flu.

The story was taken by many as fresh evidence in favor of the lab leak theory. Pundit Nate Silver confidently tweeted that, in light of this revelation, he now considered a lab leak more likely than a natural origin for Covid-19.

In fact, the Journal’s reporting actually made the case for a lab leak weaker, relative to what the State Department had previously claimed. On January 15, the department published a “fact sheet” asserting that “several researchers” at the Wuhan Institute of Virology “became sick” with respiratory symptoms in “autumn” of 2019, “before the first identified case of the outbreak.” The Journal clarified that the number of allegedly sick researchers was just three people and established that they sought treatment in mid-November, i.e., the beginning of flu season—rather than earlier in the fall. The Journal didn’t say that anyone was hospitalized and instead noted that it is common in China to seek minor medical care at a hospital instead of going to a family doctor. So we’re left with a story of three people going to the doctor with flu-like symptoms during flu season.

WIV leadership told World Health Organization investigators probing the origins of Covid that they had a normal amount of seasonal illness among their staff in the fall of 2019. But like other biosafety labs, the WIV collects and freezes yearly serum samples from the people who work there, and WIV officials told the investigators that serum samples for all staff and students in the bat coronavirus group subsequently tested negative for Covid antibodies. We have only their word to go on because the lab hasn’t been independently audited, but if that’s true, it’s a devastating blow to the lab leak theory—since, if Covid-19 escaped a lab, by far the easiest way for it to do so would be in the body of an infected staffer. Even if the Wuhan Institute of Virology is lying and some of its people had Covid in mid-November, that wouldn’t prove they caught it at work because the best available evidence suggests, by then, Covid was already circulating in Wuhan.

The problems with the lab leak theorists’ case, however, extend well beyond their overreliance on the ambiguous stories of WIV staffer sickness. Put simply, their narratives of viral sampling, evolution, and transmission don’t add up.

Dr. Angela Rasmussen, a virologist at the University of Saskatchewan who has published extensively on emerging viruses, including Covid, MERS, and Ebola, has taken considerable heat on Twitter for arguing that Covid is likely of natural origin.* She first takes aim at the popular version of the lab leak theory that posits that Covid was taken from nature and escaped in its wild form. The problem with that scenario, she told me, is that a swab from a bat contains very little infectious virus. Each bat weighs less than half an ounce, and each sample is basically a Q-tip swiped briefly over a bat’s mouth or anus. These samples are stored in vials in the freezer; they’re not likely to spill or leak, the way disaster movies have primed us to suppose.

These samples are not like huge vials of blood,” Rasmussen said. “It’s not like a big Erlenmeyer flask of green liquid.” Researchers would have to grow the virus in cells in order to stand a real chance of infecting people, she added, and it’s difficult to grow viruses from these swabbed samples even if you try to. There’s not much virus in them, and what you get tends to be contaminated with virus-killing detritus. “Technically it’s very challenging to directly isolate virus from field samples from wild animals. So that makes it unlikely that just handling those samples would result in some kind of infection.” Finally, Rasmussen added, the chemical solution that’s used to stabilize the viral RNA for sequencing is a very potent disinfectant its own right.

Then there are those who find it deeply suspicious that virus hunters haven’t produced any wild Covid-19 yet. Chinese scientists have already tested over 80,000 samples from animals, according to the first report of the WHO-China Joint Inquiry into the origins of the pandemic, but have yet to find Covid or a plausible direct ancestor in the wild. Lab leak proponents often bring this up as an argument against a natural origin. If, after all these months, investigators still can’t find Covid in nature, the reasoning goes, then maybe it’s not natural after all. “80,000 animals have been sampled since, with not one shred of connection to Covid found,” author James Surowecki scoffed on Twitter. Similar reasoning led Trump’s former Food and Drug Administration commissioner, Scott Gottlieb, to suggest on Face the Nation last month that the case for a natural origin of Covid has weakened.

Eighty thousand samples sounds like a lot, but it’s nothing in a country like China, which boasts over 7,500 native species of vertebrates. Besides, many of the animals tested in the first wave were pigs, chickens, and cows from farms across China, exotic farmed animals, and zoo animals. And since scientists often sample multiple bodily fluids per animal, 80,000 samples doesn’t even mean 80,000 individual animals. Critically, these samples appear to have been taken without any known or suspected link to early human cases of Covid. The secret to viral detective work is shoe-leather epidemiology: You’re most likely to find the earlier hosts if you start with early human cases of the mystery virus and take samples from their pets, their livestock, etc.—not from random populations.

Tracking a virus to its source can take years, and there’s no guarantee of success. It took 15 years to trace SARS definitively to bats. Failure to find a natural reservoir for a disease is not evidence of a nonnatural origin: Ebola has been around for over 40 years, and scientists are pretty confident that bats are its natural reservoir, but nobody has ever been able to culture Ebola from a bat.

If you can’t follow the path from early sick people to the sick animals in their lives, the search becomes less like detective work and more like playing the lottery: Even if a species is a viral reservoir, the virus might only be found in a tiny subpopulation. An isolated bat roost may have brewed a viral vintage found nowhere else in the world. Scientists had an edge in finding the intermediate host for MERS because the seroprevalence—the percentage of camels that carry MERS—is unusually high. Up to 100 percent of some camel cohorts tested positive for the virus. Once scientists heard stories about MERS patients having contact with camels, all they had to do was dip into various camel populations and pull up some MERS.

The Covid-19 case is much trickier. Any detective will have a better shot at cracking a case if they start with a fresh crime scene. That’s what SARS investigators had to work from, given that two of the earliest cases were a waitress and a customer at a civet restaurant, and all the restaurant’s remaining civets tested positive. The civets were still at the scene of the crime, so to speak. With Covid-19, we don’t have a crime scene. The virus first made headlines because of an outbreak at the Huanan Wholesale Seafood Market, but it turned out that there were earlier cases with no known ties to the market. In fact, there’s no reason to assume that the spillover to humans happened anywhere near Wuhan. The first human case of Covid could have been infected hundreds of miles away, perhaps closer to the horseshoe bat caves of southern China.

And it’s not as if bat coronavirus hunters are coming up empty. New SARS-like bat coronaviruses are being found all the time, in addition to the hundreds of similar viruses that were already known to science. Experts have been warning us for years that any one of these common, natural bat viruses could mutate and start a pandemic, since bat colonies are constantly producing new SARS-type coronaviruses. One four-year study of wild Chinese horseshoe bats found that just over 9 percent of the animals carried a SARS-type coronavirus. The adorable flying mammals who give us tequila are perfect viral incubators: They like to roost with other species of bats, and their relatively long lives give viruses plenty of time to mix and match. Interspecies jumps—even between kinds of bats—are a great way to select for new mutations.

China’s economy has grown explosively over the past 25 years. Development is bringing people closer to bats and other wildlife through deforestation, mining, construction, and even bat cave tourism. The $80 billion wild animal trade incentivizes trappers to spelunk around in remote areas where they may encounter bats or animals infected by bats. Moreover, demand for wild animals as luxury items brought a steady stream of animals from the rural south to major cities, including Wuhan. So it’s not that surprising that the first major outbreak of Covid-19 happened in Wuhan, a city of 11 million spread over 3,200 square miles, which is known as the Chicago of China because it so accessible by air, rail, road, and water. You can get on the fast train in Wuhan and be in Guandong Province, the home of the original SARS outbreak, in under four hours.

While we can’t rule out the possibility that Covid-19 was genetically engineered, even leading lab-leakers agree that it bears no apparent signs of genetic manipulation. It looks like a perfectly natural virus. And to alter a virus in a way that leaves no traces, you have to start with a virus that’s extremely similar to the virus you end up with.

Even if youre doing the most sophisticated gain-of-function research you could possibly be doing, you have to start with a virus thats at least close. We would estimate 99 percent, [or] even higher than that, 99.9,” Dr. Robert F. Garry, an expert on the molecular mechanisms of viral pathogenesis at the Tulane University School of Medicine and a co-author of an influential paper arguing for the natural origins of Covid, told the podcast This Week In Virology.

Furthermore, he told me by email, there’s no way to use laboratory tricks to overcome this need for a close source virus. Even if researchers were to cut and paste different natural viruses together, each component virus would have to be in the 99 percent similarity range.

There is no evidence that the Wuhan Institute of Virology, or anyone else, ever had any strain that similar to Covid-19. Lab leak boosters argue that the WIV is highly suspect because it was doing risky gain-of-function research with bat coronaviruses. But if it didn’t have wild viruses almost exactly like Covid-19, it couldn’t have engineered it, period.

As far as anybody knows, the closest strain the WIV had is a bat virus called RaTG13 that’s 96 percent similar to Covid-19, but the gulf between 96 percent and >99 percent is vast. The two viruses probably shared a common ancestor between 25 and 65 years ago, which is practically geological time for fast-mutating viruses. 

If RaTG13 were used as a backbone for Covid-19, Rasmussen told me, youd expect to see big chunks of exact similarity with coherent chunks of new information added in—like a student cutting and pasting a few original paragraphs into a plagiarized essay. Instead, she explains, Covid-19 differs from RaTG13 by over 1,000 point mutations spread through the virus like raisins in a pudding. Nobody knows what any of these little mutations do; most of them probably don’t do anything. They look like the genetic noise that accumulates in 50 years of viral evolution. They’d be a nightmare to clone in by hand, and there would be no reason to do so.

Rasmussen notes another major piece of new evidence against a lab origin: If Covid-19 were invented as part of a benevolent gain-of-function experiment, the goal would be to make it more transmissible, or more lethal to people, in order to study that strain in the lab. But when current strains of Covid-19 are cultured in cells in the laboratory, the virus tends to mutate fast and become less contagious to humans.

“When you work with this particular virus in the lab, it becomes less capable of being a human pathogen and becomes more of a cell culture adapted virus,” Rasmussen says. “So that suggests, again, that it’s unlikely that this virus, if it were being passaged extensively in cell culture, would jump out of cell culture into people and start a pandemic.”

What we’re left with is this: If the WIV had a secret strain (or strains) at least 99 percent similar to Covid-19, it got that raw material from the wild. That would mean there’s at least one wild virus that’s at least 99 percent similar to Covid-19 somewhere in nature, where humans had contact with it at least once. So far, it hasn’t been found, but it’s got to be out there, whether Covid is 100 percent natural or human-tweaked. So, given that Covid (or its direct ancestor) must exist in nature, it’s more likely that it got out naturally (like SARS and MERS) than that it took an undetectable detour through a secure biolab.

If Covid-19 were bioengineered, that would mean the WIV lab found the now-untraceable Covid-19 precursor strain(s), and even though its main job is publishing about the cool viruses it finds, it never published it or talked about it, not even to the small army of American and international scientists it collaborates with. Then it embarked on a painstaking process of undetectably tweaking the Secret Ancestor into Covid-19, its manipulation succeeded, and then multiple layers of biosecurity failed, and Covid-19 escaped.

It’s not impossible. But it involves a number of exceptions to rules—a number of carefully designed systems failing. Meanwhile, the natural origin theory just involves countless bat roosts with millions of bats doing what they do best: generating new viruses like the world’s most chaotic supercomputer.

All theories of the origins of Covid-19 should be investigated, including lab origin theories. We should go wherever the science takes us. In mid-May, 18 respected scientists with relevant expertise published an open letter in the journal Science arguing that both zoonotic and laboratory origin hypotheses “remain viable.” This letter was seized upon as additional grounds to support the lab leak theory, seemingly by people who hadn’t read it very carefully. The authors didn’t offer any new evidence, or even an argument, for why the lab leak theory deserves to be taken more seriously. Their focus was criticizing China’s stranglehold over the raw data on the origins of the pandemic and calling for a more transparent investigation. 

These are entirely valid criticisms, but on their own, they don’t move the needle on the likelihood of a lab leak. The fact that China is being secretive about Covid-19 isn’t evidence for any particular theory. China is a totalitarian regime that is notoriously secretive about everything. It should be noted that the origins of both SARS and MERS were shrouded in troubling official secrecy before they were confirmed to be natural phenomena.

Maybe it’s comforting to think that Covid-19 was carelessly released or even deliberately engineered by a handful of hubristic scientists. That seems like a relatively easy problem to control. We could ban risky research or tighten up biosecurity protocols and the problem would be solved.

By contrast, in order to deal with the ongoing natural virus threat from bats and other animals, we have to deal with much tougher problems. We have to deal with deforestation, climate change, and the international wildlife trade. We’ll need to address agricultural practices, including at home (which, after all, is where swine flu began). All this will require massive scientific and social cooperation, domestically and internationally. In the absence of hard evidence, careless lab leak speculation or overstating the case for a lab leak relative to other origin stories amounts to the casual slander of distinguished Chinese scientists. Reckless allegations undermine the research, the international relationships, and the policymaking we need to fight this pandemic and future pandemics effectively. 

*This sentence has been updated to reflect Dr. Rasmussen’s current position at the University of Saskatchewan.