A “very likely” scenario was that the virus was passed to humans through an unknown animal, the report concludes, while the widely discussed idea that it leaked from a laboratory in Wuhan is described as “extremely unlikely.” Two other scenarios — that the virus was passed directly from a bat and that it was spread via frozen food — are found to lie in between these two extremes.

The results are likely to frustrate critics of the mission, who have said that it was politicized and — because of pressure from the Chinese government, which has withheld information since the earliest days of the pandemic — did not fairly consider some scenarios.

Many of the early cases of the novel coronavirus in late 2019 were linked to the Huanan Seafood Wholesale Market, in a densely populated part of Wuhan. The WHO-led team visited the market in February and analyzed the data on early cases to consider whether they had a link to the market. They failed to reach a firm conclusion about the role of the market, based on available data.

The team found only 28 percent of confirmed early cases had exposure only to the Huanan market, while a further 5 percent had exposure to the market as well as other markets. The report notes the case with the earlier known onset of the coronavirus had “no history of exposure to the Huanan market” and of the early cases, 47 percent had no history of market exposure at all.

However, the report notes that while this may imply the Huanan market was not the original source of the outbreak, many milder cases may have been missed, with those infected not seeking medical attention. “No firm conclusion therefore about the role of the Huanan Market can be drawn,” the authors wrote.

The report also noted there were 92 cases of patients with covid-19-like symptoms in October 2019, two months before any confirmed infection. While the Chinese government has ruled these out as possible covid-19 cases, citing retrospective serological testing, the WHO team stated that levels of antibodies may have faded over the long time frame as tests were performed a year after possible infection.

The closest known relative of SARS-CoV-2 (the scientific name for the coronavirus) has been found in bats in China’s Yunnan province. However, the report notes that as the evolutionary distance between the two viruses is “estimated to be several decades,” another animal might have served as an intermediary in passing the animal to humans.

While the report concludes this is the most likely scenario, there is no direct evidence of what this animal might have been. Highly similar viruses have been found in pangolins, an endangered creature that looks like a cross between an anteater and an armadillo, but there was not stronger evidence that they served as the intermediary host for SARS-CoV-2.

The report notes the “high susceptibility of mink and cats” to the virus discovered during the pandemic suggests other animals could serve as a potential reservoir for SARS-CoV-2.

During a briefing on the Huanan Seafood Wholesale Market, the researchers were told no live animals were sold at the market and no animals were butchered on the premises, though some stores sold live fish and amphibians including turtles. Most products were frozen, the report stated, and animal products sold at the market included those from porcupines, bamboo rats, deer and several types of crocodile.

In response to the pandemic, China banned the sale and consumption of wild animals for food in February 2020. However, the trade in wild-animal products for fur and traditional Chinese medicine continues.

The report says more detailed research into what was being sold at the market needs to be performed, as well as research on any stray dogs and cats that may have made their homes near the market. The report notes that a nonaffiliated researcher told them when he visited in 2014 he saw live snakes and raccoon dogs for sale at the market.

Chinese officials have repeatedly suggested the virus might have spread to Wuhan via frozen food. The WHO-backed team investigated this scenario in detail, noting many of the goods sold at the Huanan Seafood Wholesale Market were frozen and many prior virus outbreaks have been linked to the foodstuffs.

The report also gives some credence to Chinese claims that the coronavirus has been reintroduced to the country via frozen-food imports in the year since the outbreak began in Wuhan, stating the virus can “persist in conditions found in frozen food, packaging and cold-chain products.” The report says it is “possible” frozen food led to the outbreak in Wuhan in late 2019, with the Huanan market one potential route.

But the report also states that such a scenario would have been “extraordinary” in 2019 unless the virus was already circulating widely in another location and that there is no clear evidence it was. “The probability of a cold-chain contamination with the virus from a reservoir is very low,” the report states.

The most controversial mainstream theory about the origin of the coronavirus is that it leaked from a laboratory in Wuhan in the course of scientific research. The theory has some high-profile advocates, including former Centers for Disease Control and Prevention chief Robert Redfield, but many scientists say the idea is politically motivated and not supported by direct evidence.

The WHO-backed team considered the scenario but concluded it was “extremely unlikely” and devoted only a comparatively small space in the final report to the idea of an accidental laboratory leak.

In reaching this conclusion, the report acknowledges that lab leaks do happen and that the Wuhan Institute of Virology (WIV) had collected the coronavirus strain that was most similar to SARS-CoV-2. It also noted that a Wuhan Center for Disease Control lab moved to a location near the Huanan market on Dec. 2, 2019, and that “such moves can be disruptive for the operations of any laboratory.”

However, the report quickly dismisses this idea, stating the laboratories in Wuhan that worked on the virus had good safety records and the Wuhan lab “reported no disruptions or incidents caused by the move” and was not working on bat coronaviruses before the pandemic.

According to the report, Shi Zhengli, a researcher at the WIV who conducts work on bat coronaviruses, said that there had been no infections among staffers during three years of research and that serum samples found no infections among close contacts of staffers. It is not clear whether the team asked for or was offered additional documentation on her claims.