The Covid-19 pandemic origins report looked into four hypotheses as to how the virus entered the human species, ranking them from the most to the least likely.
The final report, a joint WHO-China study, which AFP obtained Monday ahead of its official release, spelled out how the joint team of Chinese scientists and international experts thrashed out the hierarchy of probability.
Here are the four theories, in descending order of probability:
– Intermediate host animal –
This hypothesis, deemed by the experts as a "likely to very likely pathway", argues that the virus first spread from the original host animal, likely a bat, to another intermediate host animal before being passed on to humans.
— Arguments for:
Although the closest-related viruses were found in bats, the evolutionary distance between those bat viruses and the SARS-CoV-2 virus that causes Covid-19 disease is estimated to be several decades, suggesting a "missing link" in between, the report said.
Highly similar viruses have also been found in pangolins, suggesting cross-species transmission from bats at least once.
The report also pointed out that an intermediary step involving an amplifying host has been observed for several other viruses.
— Arguments against:
While SARS-CoV-2 has been found in a growing number of animal species, studies suggest they were infected by humans.
And so far, tests of a wide range of domestic and wild animals in the region where the outbreak first started have shown no evidence of SARS-CoV-2.
— Next steps:
The experts suggested the virus might have been introduced through imports to Wuhan of meat from wildlife farms in provinces where bats have been shown to carry similar coronaviruses.
"While this does not prove a link, it does provide a meaningful next step for surveys," the report said.
– Direct transmission –
This hypothesis, deemed "possible to likely", assumes that SARS-CoV-2 jumped directly from the original host, likely a bat, to humans.
— Arguments for:
Most current human coronaviruses come from animals, the report said.
Surveys, it said, had found viruses with high genetic similarity to SARS-CoV-2 in Rhinolophus bats.
It also pointed out that "antibodies to bat coronavirus proteins have been found in humans with close contact to bats".
Similar viruses have been found in Malayan pangolin, and mink have proven also highly susceptible, it said, adding it could not rule out that minks might be the primary source.
— Arguments against:
Though the closest genetic relation to SARS-CoV-2 is a bat virus, analysis indicates "decades of evolutionary space" between them, suggesting an intermediate host route is more likely.
"Also, contacts between humans and bats or pangolins are not likely to be as common as contact between humans and livestock or farmed wildlife."
— Next steps:
Trace-back studies of the Wuhan markets' supply chains provided some "credible leads", which should be expanded to other countries, said the report.
– Cold food chain –
This hypothesis, which was deemed "possible", suggests that frozen food products or their packaging might have been a route of introduction and transmission of SARS-CoV-2.
— Arguments for:
China witnessed some outbreaks related to imported frozen products in 2020.
SARS-CoV-2 has been found on the outer package of imported frozen products, suggesting the virus can persist on contaminated frozen products.
— Arguments against:
"There is no conclusive evidence for foodborne transmission of SARS-CoV-2 and the probability of a cold-chain contamination with the virus from a reservoir is very low," the report said.
— Next steps:
The experts called for screening of leftover frozen cold-chain products, especially farmed wild animals, sold in Wuhan's Huanan market from December 2019, if still available.
– Laboratory leak –
This hypothesis, which was found to be "extremely unlikely", considers that SARS-CoV-2 was introduced through a laboratory incident.
The experts examined only the theory that the natural virus escaped a lab through the accidental infection of staff. It did not consider the hypothesis of deliberate release or deliberate bioengineering of the virus, which scientists have already ruled out.
— Arguments for:
"Although rare, laboratory accidents do happen", the report said.
And CoV RaTG13, the closest strain to SARS-CoV-2, found in bat anal swabs, had been sequenced at the Wuhan Institute of Virology.
— Arguments against:
"There is no record of viruses closely related to SARS-CoV-2 in any laboratory before December 2019, or genomes that in combination could provide a SARS-CoV-2 genome," the report said.
"The risk of accidental culturing SARS-CoV-2 in the laboratory is extremely low," it added.
— Next steps:
"Regular administrative and internal review of high-level biosafety laboratories worldwide. Follow-up of new evidence supplied around possible laboratory leaks."
Which animals could have passed Covid-19 to humans?
Paris (AFP) March 29, 2021 –
First it was snakes, and then the endangered pangolin before Asian ferret badgers were put in the dock.
Scientists have been scrutinising a Noah's Ark of animals to find out whether — and how — the coronavirus was transmitted from bats to humans, with the prime suspect changing from one study to another.
Cats, dogs, badgers, lions and tigers have also been in the spotlight — not to mention minks, which have been culled in the millions.
After AFP published findings of a report by experts convened by World Health Organization (WHO) Monday, here is a recap of the suspects.
– Snakes –
Scientists were quick to accuse the bat of being the origin from the time the virus emerged in China in late 2019.
A study sponsored by the Chinese Academy of Sciences in January 2020 found that Covid-19 was closely related to a strain that exists in bats, which would be the "native host".
Bats are hosts for many other strains of coronavirus.
But the scientists say that Covid-19 must have passed through another yet-to-be-identified species known as an "intermediate host".
A second study published shortly after in the Journal of Medical Virology fingered snakes as the possible culprit.
The report was immediately brushed aside by other experts who said the guilty party was probably a mammal, as was the case with SARS, which came from the civet, a small nocturnal animal prized in China for its meat.
– The pangolin? –
Researchers at the South China Agricultural University said in February 2020 the endangered pangolin, a mammal whose scales are used in Chinese medicine, may be the "missing link" between bats and humans.
This anteater was one of the wild animals sold at the Huanan market in the central Chinese city of Wuhan, to which most of the first known cases of Covid-19 were linked.
But whether the pangolin is the culprit is not known at this stage.
– Cats and dogs vulnerable –
A pet dog was quarantined in Hong Kong later that same month after it tested "weak positive" to the virus when its owner was infected.
Cases were then reported in cats.
Ferrets and hamsters have also tested positive, along with tigers and lions in captivity.
Scientists have stressed that domestic animals are vulnerable to the virus but cannot infect humans.
– Millions of minks culled –
Suspicion has also fallen on mink, which are bred for their valuable fur.
In June, the WHO said that Dutch workers apparently infected with the coronavirus by minks could be the first known cases of animal-to-human transmission.
Cases of Covid in mink farms were then detected in several other European Union countries including Denmark, France, Greece, Italy, Lithuania, Spain and Sweden, as well as in the United States.
In July tens of thousands of minks were culled in the Netherlands and a month later hundreds of thousands more followed when the government brought forward a total ban on the industry to the end of the year.
Denmark — which had three times more mink than people — ordered all of the country's 15 to 17 million minks to be culled in November.
Copenhagen warned that the mutation via the mink, dubbed "Cluster 5", could threaten the effectiveness of any future vaccine.
– Missing link –
A mission of international WHO experts who visited Wuhan had no shortage of suspects, from rabbits to ferret badgers to raccoons and civets.
Their long-awaited report obtained by AFP Monday said it was "likely to very likely" that the virus jumped from bats to humans via an intermediate host, but they were not able to say what that missing link might be.
In fact it was also "possible to likely" that the virus jumped directly from bats, they added.
