A highly controversial hypothesis holds that SARS-CoV-2, the virus responsible for the COVID-19 pandemic, originated from a laboratory. The scientific consensus is that the virus is not the result of genetic engineering; instead, most scientists believe it spread to human populations through natural zoonotic transmission from bats, similar to the SARS-CoV-1 and MERS-CoV outbreaks and other pandemics throughout human history. Available evidence indicates that SARS-CoV-2 was originally harbored by bats and transmitted to humans through infected wild animals serving as intermediate hosts at the Huanan Seafood Market in Wuhan, Hubei, China, in December 2019. Several candidate animal species have been identified as potential intermediaries. There is no evidence supporting laboratory involvement, no indication that the virus existed in any lab prior to the pandemic, and no record of suspicious biosecurity incidents.
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 are typically first noticed in large cities, so any outbreak in China is likely to occur 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 are 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. Some scientists agree a lab leak should be examined as part of ongoing investigations, though 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), which is supported by available evidence. The origin of COVID-19 is not definitively known, and the theory it became infectious to humans through escape from a laboratory where it was being studied has not been definitively ruled out, however most scientists believe zoonosis is more likely, with a minority favoring the lab leak theory. Arguments used in support of a laboratory leak, as distinct from the lab leak hypothesis itself, have been described as characteristic of conspiratorial thinking. A 2024 survey of 168 experts, most of whom were epidemiologists and virologists, found on average they viewed zoonosis as 77% likely and a lab leak as 21% likely.
Zoonosis Most new infectious diseases begin with a spillover event from animals. These spill overs occur 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 National Institute of Allergy and Infectious Diseases's former director Anthony Fauci 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 is located within the same city as focal point of the pandemic, Wuhan's Huanan Seafood Wholesale Market, and this shared location has made it easy for conspiracy theories to take root asserting that 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. Furthermore, they observe that it is ironic lab leak proponents are keen to argue for the significance of proximity of laboratories to the outbreak, while ignoring the proximity of wet markets, which have long-been identified as potential origins for viral spillover events.
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 molecular biologist Richard Ebright) have characterized as gain-of-function. Others (including epidemiologist 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 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.
Psychological and social factors