A four year WHO-sponsored investigation of the origins of the COVID pandemic by an international group of experts has concluded that “most scientific data and accessible published scientific evidence” support the hypothesis that the novel SARS-CoV2 virus first entered the human population either directly from virus-carrying bats, or from bats to humans via intermediate hosts.  

But the possibility that the virus escaped from a lab leak remains on the table, the Scientific Advisory Group for the Origins of of Novel Pathogens (SAGO) said in a press conference just ahead of the publication of its final report on Friday.

Large gaps in data provided by China, as well as a lack of access to key United States and German intelligence reports, have confounded investigation of the lab leak hypothesis, the 27-member report of international experts concluded.  

“ Much of the information needed to assess this hypothesis has not been made available to WHO and SAGO, despite repeated request to the government of China, and therefore this hypothesis could not be investigated or excluded,” said Dr Marietjie Venter, SAGO chair and Distinguished Professor at the University of the Witwatersrand, South Africa. 

“Data provided in intelligence reports was also assessed, but tended to be very speculative, based on political opinions and not backed up by science,” Venter added. 

“SAGO and WHO have requested further information from member states, including the government of China, Germany and the United States of America, on an unpublished [intelligence] report that has been reported in the press. However, at the time of writing the required information, has not been provided to WHO or SAGO.” 

“The work to understand the origins of COVID-19 is unfinished,” said Maria Van Kerkhove, WHO’s technical lead during most of the pandemic. “We really need official and further information from China, because this is where the first cases were reported.

“This is a global search, but it really is important to make sure that the comprehensive studies that need to be done in China, where those first cases were reported, are conducted. More work needs to be done,” she said.

Zoonotic transmission via the Wuhan live animal market or another source?

In terms of the most plausible hypothesis, questions also remain as to whether SARS-CoV2 first infected humans via animals in the Wuhan live animal market, or whether the market simply became an efficient site for the virus transmission after an infected human worked or visited there.

“SAGO is not currently able to conclude exactly when, where, and how SARS-CoV2 first entered the human population,” she explained, adding that “the closest known precursor strains were identified in bats in China in 2013 and in Laos in 2020.

“These strains are too distantly related to SARS-CoV2 to be the direct source of the COVID-19 pandemic,” Van Kerkhove said. “The Huanan seafood market in Wuhan, China, played a significant role in the early transmission and amplification of the virus, with 60% of early cases in December 2019 that could be traced to the market or [people who] lived in close proximity to the market with a risk of exposure to visitors or animal products from the market. No evidence exists of widespread human or animal cases prior to December, 2019, anywhere.” 

And at the same time, there are in fact two SARS-CoV2 lineages, which were identified in infected humans at the market. This suggests that there had already been prior evolution in animals or in humans, rather than a single source, she said.

“Metagenomic evidence identified several species of wildlife that were present in the market that can be considered as potential intermediate hosts and that might have infected early human cases. Comprehensive upstream investigation at the source of wildlife species trade at this market and other markets in and around Wuhan is, however, still lacking. It is therefore not yet clear if the seafood market was where the virus first spilled over into humans or if it occurred through upstream-infected humans or animals at the market.” 

As per those questions, China has shared some of this information, “but not everything that we have requested,” Venter said.  “China has provided hundreds of viral sequences from individuals with COVID-19 early in the pandemic, but more detailed information [is needed] on animals sold at markets in Wuhan, and information on work done and biosafety conditions at laboratories in Wuhan. 

Looked at four hypotheses 

The committee examined four hypotheses, including: a spillover from animals to humans, via bats or indirectly; an accidental lab leak during field investigations or due to a break in biosafety; a third hypothesis, promoted by the Chinese in the early days, that the virus was transmitted via imported frozen food products imported into China;  and a fourth hypothesis, promoted by conspiracy fans, that the virus was the product of deliberate laboratory manipulation.

The third hypothesis was ruled out, SAGO concluded, and the fourth, regarding deliberate manipulation, remains “largely unsupported by other scientific and intelligence reports,” Venter added.

SAGO will reevaluate these hypotheses, should additional scientific evidence become available. In the meantime, a zoonotic origin, with the spillover from animals to humans, is currently considered the best supported hypothesis by the available scientific data, until further requests for information are met,” she said, adding: “Until more scientific data becomes available, the origins of SARS CoV2, and how it entered the human population will remain inconclusive.” 

Image Credits: Trinity Care Foundation/Flickr, Nature .