The WHO said that its ongoing probe into the origins of Sars-CoV-2 had uncovered new information but not dramatically changed the picture of the outbreak in Wuhan, where the virus is believed to have first emerged.
“The findings suggest the lab-incident hypothesis is extremely unlikely to explain the introduction of the virus into the human population,” said Dr Peter Ben Embarek, leader of the WHO investigation.
It had been claimed that scientists at the Wuhan Institute of Virology (WIV) were experimenting with a virus genetically similar to Sars-CoV-2 before it was leaked into the surrounding community.
China has strongly rejected that possibility and has promoted other theories that the virus may have originated elsewhere.
The WHO said it had closely inspected the WIV, concluding that “it was very unlikely that anything could escape from such a place”.
The team has made a series of recommendations for future studies to examine the origins of the virus.
Evidence suggests that it emerged naturally in bats, Dr Embarek said, and was likely passed to humans via an unconfirmed intermediary animal host, such as a pangolin or bamboo rat.
“Since Wuhan is not a city close to this environment, a direct jump from bats to the city of Wuhan is not very likely,” he added.
He said the team has potential leads of other animal species to follow when looking at the supply chain to the Huanan seafood market, where cases of Covid-19 were first traced to in December 2019.
“The search for the possible route of introduction through different animal species, and the specific reservoir, are still a work in progress,” Dr Embarek said.
He also called for the creation of a global, integrated database so that information on epidemiological and molecular data can be rapidly accessed during future outbreaks.
“We need to conduct more surveys into animal species that could act as a reservoir – and not only in China, because a lot has already been tested here,” the WHO chief said.
“It would be interesting to know if a frozen wild animal that was infected could have been a possible introduction.
“We should also look further back in tracing the source of the frozen animal products in particular that were sold in the market in December 2019.”
Professor Liang Wannian, head of the Chinese envoy working on the investigation, said the virus can survive for long periods at refrigerated temperatures and may have been introduced to the Huanan market via frozen goods – a theory advocated by China.
“Environmental sampling in Huanan market from the point of its closing reviewed widespread contamination of the surfaces, [which would be] compatible with introduction by people and animal products,” he said.
The WHO also said the virus was circulating in clusters of infections around Wuhan at the same time that it appeared in the city’s wet market.
“It was not just the cluster outbreak in the Huanan market; the virus also circulated outside of the market,” said Dr Embarek.
“The picture we see is a very classical picture of the start of an emerging outbreak, where we start with a few sporadic cases early on in the month of December, then we start to see small outbreaks where the disease starts to spread via clusters. We see that, among other instances, at the Huanan market.”
However, Prof Wannian said that the first detected case in Wuhan had no link to the market, indicating that it may not have been responsible for the city’s earliest outbreak.
“The onset date of the first case in this research was 8 December 2019,” he said. “The earliest case associated with Huanan seafood market was 12 December. The case with its onset on 8 December had no link to Huanan Market.”
However, the WHO “did not find evidence of large outbreaks" in Wuhan or elsewhere before December 2019.
“Based on analysis of this and other surveillance data, it is considered unlikely that any substantial transmission of Sars-Cov-2 infection was occurring in Wuhan in those two months [prior to discovery],” said Prof Wannian.
Professor Marion Koopmans, a virologist and member of the WHO’s investigation team, said it was not possible to say how confident the team was, in percentage terms, regarding the different hypotheses put forward for the spread of Covid-19.
The visit by the WHO team took months to negotiate after China only agreed to it amid massive international pressure at the World Health Assembly meeting last May, and Beijing has continued to resist calls for a strictly independent investigation.
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