Months after a World Health Organization inquiry concluded the novel coronavirus escaped accidentally from a laboratory in Wuhan, China, the possibility has resurfaced, giving new life to a hypothesis that many scientists regard as a conspiracy theory.
The intensified focus follows President Joe Biden's directive to U.S. intelligence organizations to "redouble their efforts" to investigate the coronavirus's origins.
On May 11, Biden's chief medical adviser, Anthony Fauci, said he was now "not convinced" that the virus evolved naturally - reversing a prior statement.
China has denied that a lab was to blame.
Virologists and scientists in related disciplines that acknowledge the probability of the virus leaking from a Wuhan lab and favor a thorough, transparent investigation disagree substantially in their assessment of either scenario.
Many believe the pandemic started with animal-to-human transmission. Others argue there isn't enough concrete information to tell which scenario is more plausible.
Another issue is whether the virus's genome sequence precludes human manipulation in a lab.
"Anytime there is an infectious disease outbreak it is important to investigate its origin," says Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins University Center for Health Security who did not contribute to the letter in Science.
"The lab-leak hypothesis is possible - as is an animal spillover," he said, "and I think that a thorough, independent investigation of its origins should be conducted."
The Wall Street Journal was the first to report new evidence: According to a U.S. intelligence report, three researchers from the Wuhan Institute of Virology became ill in November 2019 and sought medical help.
Zhao Lijian, a representative for China's foreign ministry, accused the U.S. of "hyping up the theory of a lab leak" and questioned, "Does it really care about the study of origin tracing, or is it trying to divert attention?" at a briefing in Beijing this week. Zhao also denied that three persons had become ill, as reported by The Wall Street Journal.
The organization, Biden and Fauci, among others, are calling for further investigation of the hypothesis and the gain-of-function studies on animal coronaviruses at the Wuhan lab are central to these inquiries.
Although conclusive evidence on the origins of the virus that causes COVID may never be obtained, the latest developments raise new concerns about gain-of-function experiments in general, and may lead to a reevaluation of both the experimental approach and the safety regulations of the laboratories that use it.