Viral by Alina Chan and Matt Ridley review – was Covid-19 really made in China?

Ever since the first reports of a coronavirus outbreak in Wuhan, a megacity in central China, the origins of Covid-19 have been steeped in controversy. Was Sars-CoV-2, to give the virus its official name, the result of a natural spillover event from animals to humans or was it the product of laboratory experimentation?

Although most cases clustered around a seafood market that also sold wild animals, it did not escape the attention of western intelligence agencies that nine miles to the south lay the Wuhan Institute of Virology (WIV). Nor that for many years researchers at the secure biosafety facility had been exploring caves in Yunnan, in southern China, in search of bats that harbour Sars-like viruses and isolating genetic material from their saliva, urine and faeces.

When it emerged that one of these isolates, labelled RaTG13, shared 96% of its genome with Sars-CoV-2 and that WIV researchers had been experimenting with chimeric versions capable of infecting human cells, speculation intensified. Never mind that similar “gain-of-function” experiments had been conducted at western biosecurity facilities and showed that such stepwise changes in virulence could occur naturally. The fact that the outbreak had begun in the same city as the WIV was too much of a coincidence to ignore. As two Chinese researchers put it: “The probability was very low for bats to fly to the market.”

Alina Chan and Matt Ridley are similarly puzzled why, if the virus had a natural origin, it was first detected in Wuhan and not closer to Yunnan, 900 miles to the south. “Of all the gin joints in all the towns in the world…” they write. The stakes could not be higher: if the virus was engineered in a Chinese lab and deliberately released on the world, it would amount to the crime of the century.

Wisely, Chan and Ridley do not go that far. Presenting themselves as champions of rigorous, evidence-based science, they write: “we are not alleging malfeasance, only a mistake”.

Ridley, a Conservative hereditary peer best known for his sceptical writings on climate change, and Chan, a molecular biologist at the Broad Institute in Cambridge, Massachusetts, have every right to scrutinise the evidence. Indeed, we now know their concerns were shared by several leading coronavirus experts who, at the time, publicly condemned suggestions of a non-natural origin and insisted the virus bore none of the hallmarks of being engineered. But is this the impartial and balanced inquiry the world has been waiting for?

Chan and Ridley are clever writers and in their “search for the origin of Covid-19” they venture into the deepest recesses of cyberspace, aided by an eclectic band of internet sleuths who go by such names as the “Seeker”. The result is a viral whodunnit that is sure to appeal to armchair detectives and sceptics of scientific orthodoxies.

There is just one problem: nowhere do they present proof that Sars-CoV-2 was manufactured. Take Chan’s claim that it appeared pre-adapted to human transmission “to an extent similar to late epidemic Sars”. This claim rests on a single mutation in the spike protein that appears to “slightly enhance” (Chan and Ridley’s words) its ability to bind to human receptor cells and suggests that by the time it was first detected in Wuhan it had “apparently stabilised genetically”.

But this is highly misleading. As the subsequent alphabet soup of variants demonstrates, the coronavirus has undergone repeated mutations that have steadily increased its fitness. Furthermore, studies of viruses isolated from pangolins, one of the animals suspected of being an intermediary host, bind to human receptor cells even more efficiently than Sars-CoV-2, suggesting capacity for further adaptation. As two leading virologists put it, the virus was not perfectly adapted to humans but was “just good enough”.

Another supposedly suspicious feature of Sars-CoV-2 is that it possesses a molecular key, known as a furin cleavage site, that enables it to prise open the receptor protein on human cells and kickstart the infection process. The same sequence is seen in highly contagious flu viruses and had previously been employed by researchers to modify the spike protein of Sars. Chan and Ridley suggest this is exactly the sort of insertion you would expect to find in a bat virus that had been modified in a laboratory.

However, 21 leading scientific experts recently pointed out that the furin sequence is suboptimal and that “near identical” sequences have been found in coronaviruses that commonly infect humans and cattle. In other words, although the feature is absent from known bat coronaviruses, it could just as easily be the product of natural evolution. Tellingly, Chan and Ridley do not go so far as to suggest this feature of the virus was inserted deliberately. Merely, that there is “no way to know”.

Nor is there any way of their knowing whether RaTG13 is the progenitor of the pandemic virus. This suggestion rests on the oddity that in 2013 WIV researchers recovered the bat isolate from a disused mine shaft in Mojiang, in Yunnan, close to the border with Laos, and a year earlier six men who had been clearing bat guano from the cave developed a mysterious respiratory illness and three died.

However, the 4% genetic divergence between RaTG13 and Sars-CoV-2 is equivalent to 40 years of evolutionary change. And researchers exploring caves in northern Laos have since discovered three bat coronaviruses that are more closely related and which attach to human cells more efficiently than early strains of Sars-CoV-2.

This suggests it most likely emerged naturally, either via passage through another animal host or directly via spillover from a bat, perhaps when a farmer ventured into a cave in Yunnan or Laos in search of guano. That is the most parsimonious explanation and fits with both the forensic and epidemiological evidence: samples recovered from the Wuhan seafood market are identical to human isolates and most of the original human cases had a history of prior market exposure; by contrast, there is no epidemiological link to the WIV or any other research facility in Wuhan.

Admittedly, the case for a natural origin would be stronger if scientists could produce evidence of prior infection at the Wuhan market or other Chinese wildlife markets that sold pangolins, civet cats and raccoon dogs (the most likely intermediary species). Yet although following the Sars outbreak numerous animals tested positive for sibling human viruses, in the case of Sars-CoV-2 scientists have yet to turn up any evidence of prior infections in animals. But as every schoolboy knows, absence of evidence is not evidence of absence – it may simply be that samples have yet to be taken from the right animal.

Of course, the same could be said of the absence of evidence for the lab leak theory. But no major pandemic has ever been traced to a laboratory, whereas history is littered with examples of pandemics that began as transfers of viruses from animals. In other words, a zoonotic event is the null or default hypothesis; the onus is on Chan and Ridley to demonstrate otherwise.

The tragedy is that in their desire to make a plausible case for a lab accident, Chan and Ridley neglect the far more urgent and compelling story of how the trade in wild animals, coupled with global heating and the destruction of natural habitats, makes the emergence of pandemic viruses increasingly likely. That is the more probable origin story and the scenario that should really concern us.

Mark Honigsbaum is the author of The Pandemic Century

• Viral: The Search for the Origin of Covid-19 by Alina Chan and Matt Ridley is published by Fourth Estate (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply