Why Do So Many Virologists Favour Spillover?
Marion Koopmans, Dutch virologist and member of the WHO team that investigated the origins of COVID-19 in Wuhan. (Image created with AI)
When Marion Koopmans appeared before the Dutch parliamentary inquiry into COVID-19, the committee questioned one of the most influential scientific advisers of the pandemic. Much of the hearing focused on the chaotic first weeks of 2020, the uncertainty surrounding a new virus, and the challenge of advising governments when critical information was still missing. Listening to her testimony, one is reminded how little was known at the time and how many decisions had to be made before the facts were fully understood.
The discussion eventually turned to the origins of COVID-19. Koopmans repeated a position she has held for years: that a natural spillover from animals to humans remains the most likely explanation, even if certainty remains elusive. She also observed that within the scientific community that studies emerging infectious diseases, a large majority of researchers continue to regard COVID-19 as one of many examples of a virus crossing from animals into humans. At the same time, she suggested that the tendency to treat spillover and laboratory leak as equally plausible explanations emerged largely outside that scientific community.
The committee largely moved on.
Yet that brief exchange should have opened an entirely new line of questioning.
If most virologists continue to favour spillover, while many politicians, intelligence agencies and members of the public remain unconvinced, then understanding that difference becomes important. Why do these groups look at the same uncertainty and arrive at different conclusions? What assumptions guide their thinking? What evidence do they weigh differently?
Instead, the discussion quickly returned to more familiar ground.
That feels like a missed opportunity.
After all, Marion Koopmans was not invited merely because she had read the relevant reports. Parliament could have read those reports itself. She was invited because she possesses something much more valuable: decades of experience inside the international virology community. She knows how that community thinks about emerging diseases. She understands how it assesses risk. She has spent years participating in debates about dangerous pathogens, laboratory safety and pandemic preparedness.
Those are precisely the subjects that Parliament seemed least interested in exploring.
From a virologist's perspective, spillover is hardly an unusual explanation. SARS, MERS, avian influenza and Ebola all crossed from animals into humans. Researchers such as Koopmans have spent much of their professional lives studying precisely these kinds of events. When a new virus emerges, many naturally begin by looking for the next example in a familiar pattern.
Yet COVID-19 emerged under circumstances that many people regard as anything but ordinary.
The outbreak began in Wuhan, a city that had become one of the world's leading centres for coronavirus research. Scientists there were studying bat coronaviruses, collecting virus samples and investigating how such viruses might evolve and infect humans. Questions about laboratory safety had occasionally surfaced before the pandemic. None of these facts prove that COVID-19 originated in a laboratory. But they help explain why the laboratory-leak hypothesis has remained alive long after the pandemic itself.
This is where the hearing begins to raise eyebrows.
One might expect a parliamentary committee to ask how the international virology community viewed those circumstances. How did researchers assess the risks associated with coronavirus research before 2020? How much confidence did they place in laboratory safety systems? Were laboratory accidents regarded as realistic possibilities or as highly unlikely events? How did scientists think about gain-of-function research and other controversial forms of pathogen research?
Most importantly, why did so many virologists continue to place greater weight on spillover than on a laboratory accident?
Those questions were barely explored.
That matters because expertise and independence are not the same thing. After financial crises, societies seek explanations from bankers. After military failures, they question generals. After industrial accidents, they rely on engineers. In every case, insiders possess knowledge that outsiders lack. Yet insiders are also shaped by the assumptions, experiences and culture of their own profession.
The same may be true in virology.
Scientists who have spent decades studying dangerous pathogens understand better than anyone why such research is conducted. They know what has been learned and what future threats it may help prevent. At the same time, if a pandemic were ever linked to the type of research they consider important, the consequences would extend far beyond a single laboratory. Questions would inevitably be asked about research priorities, funding, oversight and scientific culture across the entire field.
That does not mean their conclusions are wrong.
It does mean that their assumptions deserve scrutiny.
Indeed, the more expert the witness, the more important that scrutiny becomes. The purpose of a parliamentary inquiry is not simply to establish what experts think. It is to understand how they arrived at those conclusions and what assumptions underpin them.
That was the opportunity Parliament had before it.
The hearing revealed a great deal about what Marion Koopmans thinks about the origins of COVID-19. It revealed much less about why the wider virology community thinks the way it does.
Six years after the pandemic began, the debate over spillover and laboratory leak remains unresolved. The hearing was never likely to settle it.
But it could have shed light on a different question: how one of the world's leading communities of virus researchers assesses risk, uncertainty and evidence.
Marion Koopmans was one of the few people capable of helping Parliament explore that question.
The committee never really asked her to.
Written after evaluating the public hearing of Marion Koopmans before the Dutch Parliamentary Inquiry Committee on COVID-19, 29 May 2026.
