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Science’s COVID-19 reporting is supported by the Heising-Simons Foundation.

Where SARS-CoV-2 came from before it began its 15-month rampage around the globe is the biggest pandemic puzzle of all. But an eagerly awaited report on the question released today may satisfy few readers, especially given unrealistic expectations about how quickly the source of the coronavirus could be pinpointed. Produced by an international team of scientists after a carefully negotiated visit to China, where COVID-19 was first recognized, the report concludes that the likeliest start of the pandemic was a bat coronavirus that infected another, unidentified animal and then moved on to humans.

That’s long been the favored hypothesis of many virologists, but the team convened by the World Health Organization (WHO) reports little fresh evidence to support it, and members acknowledge several other scenarios, including an accidental release from a lab, remain possible. The report does, however, lay out plenty of next steps. “We still don’t know where the virus came from, but there’s a clear plan to continue investigating,” says virologist Angela Rasmussen of Georgetown University, who was on the WHO team.

The report was jointly written by 17 international experts, selected by WHO and approved by China, and an equal number of Chinese scientists. The group had been working for months together leading up to the WHO experts’ visit in January, when they reviewed data compiled by Chinese colleagues, went to sites potentially related to the pandemic’s origin, and debated the probability of different scenarios.

The report’s most definitive conclusion is also its most controversial: that it is “extremely unlikely” SARS-CoV-2 leaked out of a Chinese laboratory that was already studying coronaviruses, the Wuhan Institute of Virology (WIV). There’s little evidence to back the lab-escape hypothesis, but some researchers have criticized the WHO team members for all but ruling out the possibility when they were not authorized to investigate it independently. The WHO experts only spent a few hours at WIV and the report’s discussion of the lab leak scenario is sparse.

That’s understandable, some researchers say. “Given all the constraints and complexities here, they have probably done what they could,” says Yanzhong Huang, a global health specialist at the Council on Foreign Relations in New York City.

Rasmussen agrees. “A team of scientists is not qualified to conduct a detailed audit of WIV’s records, or get access to institutional files, lab notebooks, databases, or freezer inventories,” she says. “Nor does the WHO have the authority to stroll into China and demand that they give them unfettered access to WIV, China CDC [Center for Disease Control and Prevention], or any other institution.”

At a briefing today on the report, WHO Director General Tedros Adhanom Ghebreyesus emphasized that further studies are needed to understand the origins of the SARS-CoV-2 virus, and criticized the access given to its international team on their fact-finding mission to China. Tedros said he expected “future collaborative studies to include more timely and comprehensive data sharing. … Although the team has concluded that a laboratory leak is the least likely hypothesis, this requires further investigation, potentially with additional missions involving specialist experts, which I am ready to deploy.”

Also today, the U.S. government and 13 other countries issued a statement that echoed Tedros’s critique and called for additional research on the pandemic’s start. “Together we support a transparent and independent analysis, free from interference and undue influence, of the origins of the COVID-19 pandemic,” the statement said. It was jointly released by Australia, Canada, the Czech Republic, Denmark, Estonia, Israel, Japan, Latvia, Lithuania, Norway, South Korea, Slovenia, the United States, and the United Kingdom.

“The international expert study on the source of the SARS-CoV-2 virus was significantly delayed and lacked access to complete, original data and samples,” the statement said. It called for a “renewed commitment by WHO and all Member States to access, transparency, and timeliness.”

Most of the report’s findings were previewed at a press conference last month and in many subsequent media interviews by WHO team members. Still, at more than 300 pages, it lays out the data that the joint team of international and Chinese experts reviewed, including studies of the Huanan Market in Wuhan with which some of the first COVID-19 cases had contact.

The report outlines four scenarios for how the pandemic could have started and assesses their likelihood:

1. Transmission of SARS-CoV-2 from an animal reservoir like bats to another host in which the virus spread before infecting humans. The team calls that scenario “likely to very likely.”
2. Direct spillover into people from an animal reservoir like bats—considered “possible to likely” by the team.
3. Spillover via frozen meat from an infected animal, a route the team labeled as “possible.”
4. A lab incident leading to the first infection—the one scenario deemed “extremely unlikely.”

China has forcefully refuted the lab hypothesis while pushing the possibility that the infection could have arrived from outside China on frozen food, and some members of the WHO international team point out that the report’s conclusions reflect what the international team and its Chinese counterparts could agree on. “This entire report is a compromise,” says Fabian Leendertz, a wildlife veterinarian at the Robert Koch Institute in Germany and part of the international team. “And in a compromise, you have to respect the other’s views.”

The report recommends a host of further studies, in particular, sampling for the virus in wildlife and in farmed animals to find a possible intermediate host. But searching for SARS-CoV-2’s original animal reservoir may be most promising, Leendertz says. “At this point, it may well have disappeared from any intermediate host, so sampling bats, in particular, is probably the most likely to yield results.”

More investigations into the earliest days of the pandemic are also needed, says Thea Kølsen Fischer, a virologist at the University of Copenhagen and part of the international team. It’s still unclear when people started to get sick. In published research, scientists have described three cases of respiratory illness in China from early December 2019 that were thought to be COVID-19.

But the Chinese contingent of the WHO probe told the international team members they no longer believed those patients had SARS-CoV-2. The first case was a 62-year-old man who developed symptoms on 1 December 2019. He appeared to respond to antibiotics but got sicker later in the month and Chinese scientists reported in a journal article in 2020 that he had “laboratory confirmed” COVID-19. Although he had no contact with the Huanan market, his wife, who was hospitalized on 26 December and tested positive for COVID-19, told investigators she did. This case and the other two were discussed at length, Fischer says, and need further study. “We did not reach 100% agreement to discard these cases.”

The Chinese team also reviewed more than 76,000 other potential early COVID-19 cases—people with fever and other symptoms of the disease—and found no clear SARS-CoV-2 infections. A study of blood samples from the Wuhan blood bank to look for early COVID-19 cases is another recommended next step—although some outsiders have expressed bewilderment this hasn’t yet been done more than 15 months into the pandemic.

From the beginning, scientists on and outside the WHO team have pushed back against the idea that a short mission would quickly pinpoint the pandemic’s origins. The politically charged environment surrounding the WHO probe hasn’t helped matters, Rasmussen notes.

The report is just a first step, Fischer adds. “It feels like I’m standing in front of this wall and I’m now holding this piece of string in my hands, but I don’t know how long that string is on the other side of the wall. Maybe it’s short, and this will be done in months or maybe it’s long and this will take years.”

*Update, 30 March, 3:30 p.m.: Comments from the WHO director general and an international statement have been added.