The World Health Organization released a report at the end of March that shares the results of a four-week trip by an international team of scientists searching for the origin of SARS-CoV-2, the coronavirus that causes the Covid-19 illness.
The 120-page report details both genetic research and interviews with labs and early Covid-19 patients. The goal of the joint investigation between 17 scientists from the WHO and 17 from China was to identify the most likely ways that the virus was introduced to humans, Erin Garcia de Jesús writes for Science News.
The report concludes, in the most likely scenario, the virus started in an animal that carries many kinds of coronaviruses, like a bat. That animal probably transmitted the virus to an intermediate host, like a mink, pangolin, civet or racoon dog, which then passed the virus to a human. The investigation found that SARS-CoV-2 was making humans sick many days before it appeared at the Huanan Seafood Market in Wuhan, Michaeleen Doucleff reports for NPR. But there, the crowded, indoor aisles provided an environment where the respiratory virus can easily spread.
“This report is a very important beginning, but it is not the end,” said WHO Director-General Tedros Adhanom Ghebreyesus in a statement. “…Finding the origin of a virus takes time and we owe it to the world to find the source so we can collectively take steps to reduce the risk of this happening again. No single research trip can provide all the answers.”
Early in 2020, researchers in China collected about 1,000 samples from the Huanan Seafood Market from the surfaces of doors, stalls, toilets and trash bins, as well as from animals at the market, including mice, stray cats and 18 other species sold there. Most of the samples that contained SARS-CoV-2 came from stalls that sold seafood, livestock and poultry, Amy Maxmen reports for Nature News.
The WHO investigators also spoke with scientists at the Wuhan Institute of Virology to address concerns that the virus may have leaked from a lab. Some experts have pointed out that the evidence both for and against the lab leak theory is the thinnest section of the report, reports Amy McKeever for National Geographic. The report does not entirely rule out the possibility, but concludes that it is “extremely unlikely” that SARS-CoV-2 escaped from a lab.
The lack of evidence may be due in part to the difficulty inherent to proving a negative. “I don’t think we’ll ever be able to provide enough evidence to convince people who are convinced that it escaped from a lab that it didn’t,” says University of California, San Diego, molecular epidemiologist Joel Wertheim says to Science News. “Even if you find a virus literally identical to SARS-CoV-2 [in animals] … they could still argue that that virus had previously been found and isolated and brought into a lab and it escaped just the way it was.”
Wertheim led research, published last month in the journal Science, that uses genetic data and computer models to suggest that the virus may have jumped from an animal to humans in mid-October to mid-November 2019.
One possibility is the virus jumped from an animal to a human at a farm that supplied animals at the Huanan Seafood Market and other markets, says University of Sydney virologist Eddie Holmes to Nature News. Chinese officials have said the market did not sell live mammals or illegal wildlife, although media reports have said otherwise.
A farmed animal would probably have been an intermediate between an initial host of the virus and humans. A strain of coronavirus called CoV-RaTG13 that is found in bats is the closest relative of SARS-CoV-2, but those bats are a thousand miles away from Wuhan, per National Geographic.
The WHO report concludes that it is “likely to very likely” that SARS-CoV-2 originated in an animal, like bats, and reached humans through an intermediate, like a farmed mammal.
The researchers tested 18 species of animals from the Huanan Seafood Market for signs of the coronavirus, and all came back negative. But the report outlines more than three dozen species of wild mammals that are bred on farms in China, per NPR.
The samples gathered for the report are “a fraction of the animals that are farmed or captured or transported for this purpose in China,” says Georgetown University Medical Center virologist Angela Rasmussen to National Geographic. “I think we haven’t done anywhere near enough sampling.”
The report recommends future studies focus on disease surveillance in captive animals and wild bats both in China and Southeast Asia. It also recommends closer epidemiological studies of the earliest Covid-19 cases.
“A lot of good leads were suggested in this report, and we anticipate that many, if not all of them, will be followed through because we owe it to the world to understand what happened, why and how to prevent it from happening again,” says the WHO’s Peter Ben Embarek, co-leader of the investigation, to Nature News.