A team of World Health Organization scientists said on Tuesday in China that the coronavirus probably first spread to humans through an intermediate animal host and was “extremely unlikely” the result of a lab accident.
The findings, delivered after 12 days of field work by the team visiting Wuhan, China, were the first step in what will most likely be a painstaking process to trace the origins of the global pandemic, a question that is critical to helping prevent a recurrence.
“All the work that has been done on the virus and trying to identify its origin continue to point toward a natural reservoir,” said Dr. Peter K. Ben Embarek, a food safety scientist with the W.H.O., who is leading the team of experts, speaking at a news conference in Wuhan, the city where the coronavirus was first discovered late in 2019.
Dr. Embarek dismissed the idea that the virus might have emerged from a laboratory in Wuhan, a theory that has gained currency among some officials and experts in the United States and elsewhere. “It was very unlikely that anything could escape from such a place,” he said, citing safety protocols.
The W.H.O. experts largely focused their comments on the scientific aspects of their mission. But the inquiry has been in many ways overshadowed by politics.
The Chinese government has continued to suggest that the virus may have originated overseas, an idea that many scientists discount. Chinese officials on Tuesday used the news conference to continue to promote this theory, arguing that the search for the origin of the virus should focus on places outside China.
The investigation will “not be restricted to any location,” said Liang Wannian, who led the team of Chinese scientists assisting in the W.H.O. mission.
The W.H.O. experts at the three-hour news conference did not challenge the statements by the Chinese officials. They pledged to examine reports of early cases of the virus outside China. They also called for more research into the animals that were sold at a sprawling market in Wuhan where some of the first cases of the virus were detected.
For the W.H.O., the visit served as a chance to dispel criticism that it is too deferential to China.
For months, experts and politicians have denounced the W.H.O. for allowing the Chinese government to control the inquiry into the source of the pandemic. Chinese officials, wary of drawing attention to missteps during the outbreak, repeatedly delayed the visit by W.H.O. experts and sought to limit the scope of their mission. The Chinese government acquiesced to growing global pressure and finally allowed the team of 14 scientists into Wuhan last month.
The W.H.O. has used the inquiry to project an image of transparency and independence. While in Wuhan, the team of scientists used social media to chronicle their visits to laboratories, disease-control centers and live-animal markets. W.H.O. officials have vowed to ask tough questions and to push for access to data and research, but it remains unclear how forthcoming the Chinese government will be.
“If the team doesn’t come up with something of substance, there’s also the risk of people saying this was all just a show,” said Daniel R. Lucey, an infectious-disease specialist at Georgetown University.
The inquiry is proceeding in a charged political environment. The Chinese ambassador to the United States, Cui Tiankai, recently suggested that the United States should allow the W.H.O. to send investigators there as part of its inquiry.
Officials in the United States and other Western countries have at times cast doubt on the independence of the W.H.O. investigation, worried that China may try to influence the results.
Now, it will fall on the team of scientists to carry out a difficult investigation that many says could take months or even years.
While China ultimately cooperated with the visit, the government is still firmly in control of research related to the virus in China and may try to prevent the release of embarrassing information.
“One visit is not enough time to do a thorough investigation,” said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations. “They’re doing all the work within the parameters set by the Chinese government.”
The W.H.O. team has praised the Chinese scientists assisting in the mission, saying the government worked in good faith to grant access to important sites such as the Huanan Seafood Wholesale Market, where some of the earliest cases were detected. The scientists were also allowed to visit the Wuhan Institute of Virology, which houses a state-of-the-art laboratory that has been at the center of several unsubstantiated theories about the virus.
Albee Zhang contributed research.