Private messages sent by Andersen, the lead author of an influential early paper dismissing the laboratory origin theory of COVID-19, have surfaced, showing him expressing significant uncertainty about that conclusion months after the paper was published.
Andersen, a professor at Scripps Research and a co-author of the paper, wrote in a Slack message dated June 11, 2020, that he was troubled by evidence he was encountering. Andersen described what he called smoking guns and said he was not comfortable with what was emerging. Andersen also wrote that he was struggling with the fact that the authors did not have hard evidence pointing definitively in either direction, and that they could not rule out the possibility that someone had deliberately inserted the furin cleavage site, a distinctive feature of the virus, into SARS-CoV-2.
The furin cleavage site has been one of the central points of scientific and political debate about the origins of the virus. It is a structural characteristic that enhances the ability of the virus to enter human cells, and its presence in SARS-CoV-2 at a specific location was noted early in the pandemic as unusual. His private message referred to research conducted in 2006 that explored inserting such a feature at the same location in a related virus.
What the published paper said
The paper The Proximal Origin of SARS-CoV-2, published in Nature Medicine in March 2020, stated that the authors’ analyses clearly showed the virus was not a laboratory construct or a purposefully manipulated virus, and that no lab-based scenario for its origin was plausible. The paper became one of the most frequently cited early pieces of scientific literature used to argue against the lab leak hypothesis.
The gap between the paper’s public conclusions and Andersen’s private expressions of doubt, made months after publication, has reignited debate about how confidently the paper’s conclusions were actually held by those who wrote it.
Reports indicate that another Andersen co-author assigned a 20 percent probability to the lab leak hypothesis at around the same time, a figure he later revised downward.
Congressional attention
The messages drew immediate response from members of Congress who have spent years pressing for more transparency on the question of COVID-19’s origins. One senator who has been among the most vocal critics of how federal health officials handled the origins question cited the private messages in a public statement, noting the contrast between the probability estimates the authors reportedly assigned in private and the confident public dismissal they offered in the published paper.
The senator framed Andersen’s internal probability estimates, which he described as ranging from 10 to 30 percent, as meaningful evidence of actual uncertainty, arguing that those figures were far from Andersen’s definitive ruling-out language that appeared in the published literature and that shaped public debate.
The ongoing debate
The question of where SARS-CoV-2 originated remains officially unresolved. Multiple U.S. intelligence agencies have assessed the lab leak hypothesis as plausible, though they have reached different conclusions about which explanation is more likely. A small number favor a natural origin. Others lean toward a laboratory-related incident. None have concluded the question with certainty.
Scientists who study coronaviruses and the origins of infectious disease remain divided. Some argue that natural spillover from animals to humans is the most parsimonious explanation consistent with available evidence. Others point to the particular characteristics of SARS-CoV-2, including the furin cleavage site, as features that warrant serious investigation of research-related origins.
The release of private communications among scientists has become a recurring element of the ongoing debate, with congressional investigators and researchers on multiple sides of the question using newly surfaced documents to argue that the public record understated the level of genuine uncertainty that existed among some of the scientists who shaped early public messaging on the virus’s origins.
Andersen has not publicly responded to the content of the messages.
