We are 23 of the 27 original members of the Scientific Advisory Group for the Origins of Novel Pathogens (SAGO) for the World Health Organization (WHO). After nearly 3.5 years of deliberations, we concluded our independent assessment of the origin of the coronavirus SARS-CoV-2, and provided our report1 in June 2025 to the WHO director-general.
Meet the scientists investigating the origins of the COVID pandemic
In the 78-page document, we determined that most of the peer-reviewed scientific evidence supports the hypothesis that SARS-CoV-2 has a zoonotic origin, meaning that it came from an animal. But until requests for additional information are met or more data become available, there can be no certainty about when, where and how SARS-CoV-2 entered the human population.
Although the term of the first SAGO group ended in October last year, meaning we are no longer members, the WHO has proposed a second term for SAGO and issued a call for new participants. Our 2025 report1 provides recommendations for subsequent investigations seeking to establish the origin of the COVID-19 pandemic. In the meantime, with the politicization and speculation around the origin of the pandemic showing no signs of abating, 23 of us mark the close of SAGO’s first chapter by clarifying our position on the origin of SARS-CoV-2 and the science behind it in a more accessible way.
Of the four original members of SAGO who are not co-authors of this article, one resigned before the 2025 report was finalized. Three others dissented from our decision to evaluate the possibility that SARS-CoV-2 had leaked from a laboratory on the basis of there being no evidence for this (see hypotheses three and four below). We are grateful to all four for their contributions to our investigation. One of us (M.G.G.), whose name is on the 2025 report, requests that her dissent regarding the evaluation of the lab-leak hypotheses be noted.
We stress that we are solely responsible for the views expressed in this article, and that such views do not necessarily represent the views, decisions or policies of the WHO, or of any institutions that we are affiliated with.
Mission-driven
The WHO’s director-general, Tedros Adhanom Ghebreyesus, established SAGO in November 2021 and tasked it with designing a guiding framework to steer two things: an ongoing effort to try to determine the origins of SARS-CoV-2, and investigations of how emerging and re-emerging pathogens originate more broadly2.
The 27 independent scientific experts from 27 countries who made up SAGO investigated all reasonable possibilities — not with the aim of blaming any one country for the emergence of SARS-CoV-2, but to determine what happened so that the world is better able to prevent epidemics and pandemics in the future.
To investigate the origin of SARS-CoV-2 in the human population, we assessed reports of early cases and clinical studies. We investigated potential sources of infection, including animal reservoirs, intermediate hosts, insect vectors and environmental sources. We also assessed the genomics and evolutionary biology of the virus, as well as the possibility of the virus having escaped through a lab or research-associated incident. These key ‘checkpoints’ are listed in the global framework document that we published in 20243.
A nurse and patient at a temporary COVID-19 ward outside a hospital in Los Angeles, California, in 2021.Credit: Francine Orr/Los Angeles Times via Getty
Our deliberations on the origins of SARS-CoV-2 have been based on the available scientific evidence. We used the work of international teams4 that in 2020 and 2021 had visited Wuhan, China, where the first evidence-backed cases of the virus circulating in humans were discovered5. We used published scientific studies relevant to all the areas we were investigating; preprints and presentations by scientists who had worked on relevant research, either in China or elsewhere; and discussions involving international experts and SAGO members held between November 2021 and June 2025. We also consulted reviews conducted by various scientific bodies, including a 2022 Lancet Commission6. And we reviewed publicly available intelligence reports from oversight committees and government agencies, as summarized in our 2025 publication1.
Four scientifically credible hypotheses had been proposed before November 2021 by the Joint WHO–China Study, an effort to rapidly inform China and the world on next steps in the response to the outbreak. Here, we summarize our conclusions following our investigations into the plausibility of each.
Hypothesis one: animals infected with SARS-CoV-2 passed the virus to humans
Most of the peer-reviewed scientific evidence supports this hypothesis.
Although they are too distantly related to SARS-CoV-2 to have been its direct progenitors, closely related ancestral strains have been found in horseshoe bats (Rhinolophus spp.) in southeast Asia. A Betacoronavirus called RaTG13, which shares 96.1% of its genetic code with SARS-CoV-2, was identified in 2013 in China7. (Betacoronavirus is the genus to which SARS-CoV-2 belongs.) Another strain called BANAL-52, which shares 96.8% of its genetic code with SARS-CoV-2, was identified in 2020 in Laos8. This suggests that similar strains circulating in bats in China or southeast Asia might have spilled over to intermediate animal hosts or directly to humans.
Why the world needs more transparency on the origins of novel pathogens
We also know that the Huanan Seafood Market in Wuhan had a significant role in the early transmission and initial spread of the virus.
More than 60% of the known earliest human cases in December 2019 involved people who worked at the market, made purchases there, lived near it or had some other epidemiological link with it9. (At least 175 people were diagnosed with the disease, either through lab testing or on the basis of clinical symptoms before 1 January 2020.)
Two distinct genetic lineages of SARS-CoV-2 were identified in samples from early cases associated with the Huanan market, as well as from the environmental sampling of stalls and run-off water in drains at the market in January 2020. This supports the idea that the virus had already been evolving in animals before it reached the market10.
Raccoon dogs (Nyctereutes procyonoides) are susceptible to early strains of SARS-CoV-2.Credit: Getty
Metagenomic sequencing of environmental samples collected at the market has indicated that several wildlife species had been there before it was cleaned and sterilized on 1 January 2020 by the Chinese authorities in response to the outbreak. These include raccoon dogs (Nyctereutes procyonoides), hoary bamboo rats (Rhizomys pruinosus) and palm civets (Paguma larvata), all of which are known to be susceptible to the early strains of SARS-CoV-211,12. These animals could have been the intermediate hosts that brought the virus to the market, leading to the early cases in humans11,12 — although it remains unclear whether the virus first infected humans at Huanan, or whether the spillover event occurred earlier, with the virus subsequently being carried to the market by infected humans or animals.
Adding to all this, there is no verified evidence of the existence of human or animal cases of SARS-CoV-2 infection anywhere else before December 2019. There were retrospective reports of one or two possible cases occurring in November 2019 in Italy and France, and of the virus being detected in a wastewater sample (also in November that year) in Brazil. But in all instances, the positive test results could not be confirmed by independent labs.
Hypothesis two: SARS-CoV-2 was introduced into China’s animal markets from overseas through imported goods
The 2021 WHO–China Joint Study and SAGO’s 2022 report both determined that further data would be required to support this idea4,5.
In April last year, the Chinese government endorsed this route as the most likely source of SARS-CoV-2. But almost six years on from the start of the pandemic, we conclude that the available scientific evidence does not support this hypothesis1.
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SARS-CoV-2 was detected on frozen goods, but only several months into the pandemic when the virus was already widespread in humans. By this point, infected people could have contaminated surfaces. Also, no further evidence has become available to suggest that the transmission of SARS-CoV-2 to humans from frozen products occurred at Huanan Seafood Market, at any other market in Wuhan or elsewhere at the start of the pandemic.
Hypothesis three: SARS-CoV-2 originated from an accidental lab-related event
Much of the information needed to assess this hypothesis has not been made available to the WHO or SAGO.
Repeated requests have been made to the Chinese government by the WHO to release the health records of research-lab staff, biosafety and biosecurity protocols, and audits or independent inspections conducted to verify the safety procedures of labs in Wuhan. These include those of the Chinese Center for Disease Control and Prevention in Wuhan and the Wuhan Institute of Virology, a leading research centre for bat coronaviruses. In our view, the government has not provided the necessary information that we have requested since the launch of SAGO in November 2021 to investigate a potential biosafety breach. The National Health Commission of China states that the country has shared all relevant data and information, and proposes that investigations be undertaken instead in labs in other countries where coronavirus research has been conducted13.
A bird market in Hong Kong in 2025.Credit: Yau Ming Low/Shutterstock
Most of the scientific reviews we assessed support the zoonotic-origins hypothesis and find no conclusive evidence for a lab leak. Most of the published government-agency and intelligence reports that we assessed (which are listed in our 2025 publication1) assign levels of confidence, such as ‘low’ or ‘moderate’, to the lab-leak or zoonotic-origins hypotheses. But such reports deliver different conclusions, seemingly on the basis of political rather than scientific arguments.
In short, published intelligence reports, which focus mainly on biosafety and biosecurity policies and practices at the Wuhan Institute of Virology, are speculative. None provides concrete evidence of a breach.
Together, these reports highlight the continued need for a thorough, unbiased investigation into the origins of SARS-CoV-2 to enhance global health security. SAGO and the WHO have requested additional intelligence reports, including those that were classified or unpublished at the time, which several countries have produced in recent months — including China, Germany and the United States. This information has not yet been provided to SAGO or to the public.
Hypothesis four: SARS-CoV-2 originated from the deliberate manipulation of a virus in a lab
We analysed reports on the genome structure of SARS-CoV-2, as well as publications addressing the likelihood that it had been manipulated through reverse genetics. This involves making alterations to a virus’s known genes or regulatory elements to investigate how targeted mutations change its properties. Such changes might be introduced to make a virus less pathogenic, for instance, or easier to study.
We did not find evidence to suggest that SARS-CoV-2 resulting from experimental manipulation was a more likely scenario than it emerging from naturally occurring mutations or recombination events. (Recombination involves the exchange of genetic material between genetically distinct, but usually closely related, viruses that infect the same host cell.)
Members of Sarbecovirus, the viral subgenus of the Betacoronavirus genus to which SARS-CoV-2, SARS-CoV-1 and other SARS-related coronaviruses (mainly from bats) belong, have mosaic genomes. Pieces of the viruses’ genomes originate from distinct evolutionary lineages owing to multiple recombination events in bats, or in other hosts14.
