Proposed theory on the origins of COVID-19
 
The COVID-19 lab leak theory, or lab leak hypothesis, is the idea that SARS-CoV-2, the virus that caused the COVID-19 pandemic, came from a laboratory. This claim is highly controversial; most scientists believe the virus spilled into human populations through natural zoonosis (transfer directly from an infected non-human animal), similar to the SARS-CoV-1 and MERS-CoV outbreaks, and consistent with other pandemics in human history. Available evidence suggests that the SARS-CoV-2 virus was originally harbored by bats, and spread to humans from infected wild animals, functioning as an intermediate host, at the Huanan Seafood Market in Wuhan, Hubei, China, in December 2019. Several candidate animal species have been identified as potential intermediate hosts. There is no evidence SARS-CoV-2 existed in any laboratory prior to the pandemic, or that any suspicious biosecurity incidents happened in any laboratory.
Many scenarios proposed for a lab leak are characteristic of conspiracy theories. Central to many is a misplaced suspicion based on the proximity of the outbreak to the Wuhan Institute of Virology (WIV), where coronaviruses are studied. Most large Chinese cities have laboratories that study coronaviruses, and virus outbreaks typically begin in rural areas, but are first noticed in large cities. If a coronavirus outbreak occurs in China, there is a high likelihood it will occur near a large city, and therefore near a laboratory studying coronaviruses. The idea of a leak at the WIV also gained support due to secrecy during the Chinese government's response. The lab leak theory and its weaponization by politicians have both leveraged and increased anti-Chinese sentiment. Scientists from WIV had previously collected virus samples from bats in the wild, and allegations that they also performed undisclosed work on such viruses are central to some versions of the idea. Some versions, particularly those alleging genome engineering, are based on misinformation or misrepresentations of scientific evidence.
The idea that the virus was released from a laboratory (accidentally or deliberately) appeared early in the pandemic. It gained popularity in the United States through promotion by conservative personalities in early 2020, fomenting tensions between the U.S. and China. Scientists and media outlets widely dismissed it as a conspiracy theory. The accidental leak idea had a resurgence in 2021. In March, the World Health Organization (WHO) published a report which deemed the possibility "extremely unlikely", though the WHO's director-general said the report's conclusions were not definitive. Subsequent plans for laboratory audits were rejected by China.
Most scientists remain skeptical of the possibility of a laboratory origin, citing a lack of any supporting evidence for a lab leak and the abundant evidence supporting zoonosis. Though some scientists agree a lab leak should be examined as part of ongoing investigations, politicization remains a concern. In July 2022, two papers published in "Science" described novel epidemiological and genetic evidence that suggested the pandemic likely began at the Huanan Seafood Wholesale Market and did not come from a laboratory.
Background.
The principal hypothesis for the origin of COVID-19 is that it became infectious to humans through a natural spillover event (zoonosis). That it became infectious to humans through escape from a laboratory where it was being studied is a minority position. The available evidence supports zoonosis. Although the origin of SARS-CoV-2 is not definitively known, arguments used in support of a laboratory leak are characteristic of conspiratorial thinking.
Zoonosis.
Most new infectious diseases begin with a spillover event from animals, and furthermore, they spill over spontaneously (either by contact with wildlife animals, which are the majority of cases, or with farmed animals). For example, the emergence of Nipah virus in Perak, Malaysia, and the 2002 outbreak of SARS-CoV-1 in Guangdong province, China, were natural zoonosis traced back to wildlife origin. COVID-19 is considered by scientists to be "of probable animal origin". It has been classified as a zoonotic disease (naturally transmissible from animals to humans). Some scientists dispute this classification, since a natural reservoir has not been confirmed. The original source of viral transmission to humans remains unclear, as does whether the virus became pathogenic (capable of causing disease) before or after a spillover event.
Bats, a large reservoir of betacoronaviruses, are considered the most likely natural reservoir of SARS‑CoV‑2. Differences between bat coronaviruses and SARS‑CoV‑2 suggest that humans may have been infected via an intermediate host. Research into the natural reservoir of the virus that caused the 2002 SARS outbreak has resulted in the discovery of many SARS-like coronaviruses circulating in bats, most found in horseshoe bats. Analysis indicates that a virus collected from "Rhinolophus affinis" in a cave near the town of Tongguan in Yunnan province, designated RaTG13, has a 96% resemblance to SARS‑CoV‑2. The RaTG13 virus genome was the closest known sequence to SARS-CoV-2 until the discovery of BANAL-52 in horseshoe bats in Laos, but it is not its direct ancestor. Other closely related sequences were also identified in samples from local bat populations in Yunnan province. One such virus, RpYN06, shares 97% identity with SARS-CoV-2 in one large part of its genome, but 94% identity overall. Such "chunks" of very highly identical nucleic acids are often implicated as evidence of a common ancestor.
An ancestor of SARS-CoV-2 likely acquired "generalist" binding to several different species through adaptive evolution in bats and an intermediate host species. Estimates based on genomic sequences and contact tracing have placed the origin point of SARS-CoV-2 in humans as between mid-October and mid-November 2019. Some scientists (such as Fauci above and CIRAD's Roger Frutos) have suggested slow, undetected circulation in a smaller number of humans before a threshold event (such as replication in a larger number of hosts in a larger city like Wuhan) could explain an undetected adaption period.
The first known human infections from SARS‑CoV‑2 were discovered in Wuhan, China, in December 2019. Because many of the early infectees were workers at the Huanan Seafood Market, it was originally suggested that the virus might have originated from wild animals sold in the market, including civet cats, raccoon dogs, bats, or pangolins. Subsequent environmental analyses demonstrated the presence of SARS-CoV-2 in the market, with highest prevalence in areas of the market where animals known to be susceptible to SARS-CoV-2 infection were held. Early human cases clustered around the market, and included infections from two separate SARS-CoV-2 lineages. These two lineages demonstrated that the virus was actively infecting a population of animals in the market, and that sustained contact between those animals and humans had allowed for multiple viral transmissions into humans. All early cases of COVID-19 were later shown to be localized to the market and its immediate vicinity.
While other wild animals susceptible to SARS-CoV-2 infection are known to have been sold at Huanan, no bats or pangolins were sold at the market.
Wuhan Institute of Virology.
The Wuhan Institute of Virology and the Wuhan Center for Disease Control are located within miles of the original focal point of the pandemic, Wuhan's Huanan Seafood Wholesale Market, and this very closeness has made it easy for conspiracy theories to take root suggesting the laboratory must be the virus' origin. However virology labs are often built near potential outbreak areas. Proponents of the lab leak theory typically omit to mention that most large Chinese cities have coronavirus research laboratories. Virus outbreaks tend to begin in rural areas, but are first noticed in large cities. Stephan Lewandowsky and colleagues write that the location of the Institute near the outbreak site is "literally a coincidence" and using that coincidence as "a priori" evidence for a lab leak typifies a kind of conjunction fallacy.
The Wuhan Institute of Virology (WIV) had been conducting research on SARS-like bat coronaviruses since 2005, and was involved in 2015 experiments that some experts (such as Richard Ebright) have characterized as gain-of-function. Others (including Ralph Baric) have disputed the characterization, pointing out that the experiments in question (involving chimeric viruses) were not conducted at the WIV, but at UNC Chapel Hill, whose institutional biosafety committee assessed the experiments as not "gain-of-function". Baric did acknowledge the risks involved in such studies, writing, "Scientific review panels may deem similar studies building chimeric viruses based on circulating strains too risky to pursue ... The potential to prepare for and mitigate future outbreaks must be weighed against the risk of creating more dangerous pathogens."
The fact that the lab is in Wuhan, the city where the pandemic's early outbreak took place, and the fact that the research at WIV was being conducted under the less stringent biosafety levels (BSL) 2 and 3, has led to speculation that SARS-CoV-2 could have escaped from the Wuhan lab. Richard Ebright said one reason that lower-containment BSL-2 laboratories are sometimes used is the cost and inconvenience of high-containment facilities. Australian virologist Danielle Anderson, who was the last foreign scientist to visit the WIV before the pandemic, said the lab "worked in the same way as any other high-containment lab". She also said it had "strict safety protocols". The Huanan Seafood Market may have only served as a jumping off point for a virus that was already circulating in Wuhan, facilitating rapid expansion of the outbreak.
Prior lab leak incidents and conspiracy theories.
Laboratory leak incidents have occurred in the past. A Soviet research facility in 1979 leaked anthrax and at least 68 people died. The 2007 foot-and-mouth outbreak in the UK was caused by a leaky pipe at a high-security laboratory. The SARS virus escaped at least once, and probably twice, from a high-level biocontainment laboratory in China.
Benign exposures to pathogens (which do not result in an infection) are probably under-reported, given the negative consequences of such events on the reputation of a host institution and low risk for widespread epidemics. Epidemiologist Marc Lipsitch and bacteriologist Richard Ebright have said that the risk of laboratory-acquired infection (especially with modified pathogens) is greater than widely believed.
No epidemic has ever been caused by the leak of a novel virus. The only incident of a lab-acquired infection leading to an epidemic is the 1977 Russian flu which was probably caused by a leaked strain of H1N1 that had circulated naturally until the 1950s.
Previous novel disease outbreaks, such as AIDS, H1N1/09, SARS, and Ebola have been the subject of conspiracy theories and allegations that the causative agent was created in or escaped from a laboratory. Each of these is now understood to have a natural origin. Anti-biotechnology activists falsely claimed that a plant pathogen of olive trees was the result of scientists' work, despite evidence to the contrary that the pathogen was not a laboratory strain. Studies later showed the origin was long before the workshop that was the subject of the false claims, and a more typical route of introduction by an imported plant.
Proposed scenarios.
The lab leak theory is not a single discrete proposed scenario, but a collection of various proposed scenarios on a spectrum with, at one end, a careless accident from legitimate research; at the other, the engineering and release of a Chinese biological weapon. While the proposed scenarios are theoretically subject to evidence-based investigation, it is not clear that any can be sufficiently falsified to placate lab leak supporters, and they are fed by pseudoscientific and conspiratorial thinking.
There is no evidence that any laboratory had samples of SARS-CoV-2, or a plausible ancestor virus, prior to the start of the COVID-19 pandemic.
Various sources have hypothesised that SARS-CoV-2 could have leaked from the Wuhan Institute of Virology or another laboratory in Wuhan, such as the Wuhan Center for Disease Control. The theories vary on whether this was an intentional act or an accident. Theories also vary on whether the virus was modified by human activity prior to being released. By January 2020 some lab leak proponents were promoting a narrative with conspiracist components; such narratives were often supported using "racist tropes that suggest that epidemiological, genetic, or other scientific data had been purposefully withheld or altered to obscure the origin of the virus". David Gorski refers to "the blatant anti-Chinese racism and xenophobia behind lab leak, whose proponents often ascribe a nefarious coverup to the Chinese government". Gorski later stated the lab leak hypothesis hasn't "stood up to scientific scrutiny". The use of xenophobic rhetoric also caused a rise in anti-Chinese sentiment.
On social media the idea that COVID was a Chinese biological weapon has become widespread, and accords with rhetoric about how a yellow peril threatens white people. Science historian Fred Cooper and colleagues write that