Explained | A boost to the zoonoses theory regarding the origins of the SARS-CoV-2 virus

What are the two theories with regards to the origins of the SARS-CoV-2 virus which caused the COVID-19 pandemic? What is the recent evidence which has been unearthed? What are its implications? Why is it important to know where the virus came from? Has the debate finally settled?

March 20, 2023 10:47 pm | Updated 10:47 pm IST

A woman walks in front of the closed Huanan wholesale seafood market in January 2020.

A woman walks in front of the closed Huanan wholesale seafood market in January 2020. | Photo Credit: AFP

The story so far: In the never-ending debate about the origin of the virus that set off the COVID-19 pandemic, there is now fresh grist for the mill for those who have held out for the zoonotic angle — that the infection skipped from animals to humans. Undisclosed genetic data from a food market in Wuhan has been unearthed and is being used to support the zoonoses theory over the lab leak theory. These findings were presented to the Scientific Advisory Group for the Origins of Novel Pathogens, an expert body constituted by the WHO.

What is this fresh theory about?

According to Science, Florence Débarre, who specialises in evolutionary biology at the Centre National de la Recherche Scientifique in France, discovered the data which consists of genetic sequences posted in GISAID, a virology database, by Chinese researchers.

The Chinese team had reportedly collected samples from the Huanan Seafood Wholesale Market, which has been, since 2020 notoriously connected to a cluster of early COVID-19 cases. They had, in February 2022, posted a preprint with a graph that showed several environmental samples from the market containing the virus which also had human genetic material. It was this that Debarre had stumbled upon. It also reported that these sequences were later removed on the request of the authors.

The paper also records the response of George Gao, former head of the Chinese Center for Disease Control and Prevention (China CDC), to the question why the sequences were not made public earlier: Gao said the sequences are “[n]othing new. It had been known there was illegal animal dealing and this is why the market was immediately shut down.” It goes on to document early published research from Gao and team that proposes that humans brought the virus to the market, having detected that some of the samples that tested positive for SARS-CoV-2 had human DNA, but none other. On the contrary, the evidence in front of the WHO now is that “some coronavirus-positive samples collected contained DNA or RNA from raccoon dogs, civets, and other mammals now known to be highly susceptible to SARS-CoV-2.”

Michael Worobey, Head of Ecology and Evolutionary Biology at the University of Arizona, who has been independently researching the twin theories, says the path of evidence leads up the zoonosis path, and not of the lab leak. Writing in the LA Times last week he said: “There is now a large body of peer-reviewed scientific research consistent with a zoonotic origin of this pandemic. However, there is no credible, peer-reviewed research pointing to a lab leak. Had the evidence gone in the other direction, I’d be reporting that. But it hasn’t.” Worobey and team used cellphone mobility data to indicate that the Wuhan market was an unlikely location for the world’s first cluster of COVID cases, unless it was also from where the virus emerged.

U.S.-based virologist Angella Rassmussen of the International Vaccine Centre who studies the struggle between host and pathogen, in an interview to NPR last week said: “I think we’re all open to the fact that evidence could emerge that shows that it didn’t come from the market — that it came from a lab. And I think every good scientist is going to be open to that. That’s literally our job — is to try to make our hypotheses not true, to falsify them — as my colleague says, to kick the tires of those hypotheses and see if they work. And so far, that hypothesis about the market origin has stood up.”

Is it important to analyse origins?

The main theoretical dualism that the world has contended with since the COVID-19 pandemic is the origin question — lab leak or zoonoses. In a letter in Science journal published nearly two years ago, scientists urged the scientific world to keep an open mind about both the theories, on the ground that they remained viable. They also called upon public health agencies, and researchers to make their data and analyses open to the public. It is expected that the recent genetic data that was presented to the WHO will also find its way to a peer-reviewed journal. Science quotes Worobey, who also attended the WHO meeting, as saying: “We are hoping to be able to talk more about our analyses once they’re completely wrapped up very soon.”

Is this the final word on the origin question?

Clearly, there will be many more pow wows in the future, most raucous on social media perhaps, as researchers on both sides of the dichotomous question endeavour to gather evidence to support their theory. This divisive debate will not die down; it is likely to continue until sufficient evidence emerges to arrive at a single cause.

Critics of the animal transmission theory are demanding specifics and details on the ‘spill over’ while virologists say it would be impossible to get that level of data. With post facto research alone guiding the investigation, the scrutiny will likely continue. But will there be sufficient nails to drive all speculation that is sans evidence into the ground? This remains to be seen.

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