Scientists who were among the first to speak out against the theory that the pandemic could have started from a laboratory leak have again disputed the idea and called for more collaboration to investigate the origins of the virus.
In a letter published in The Lancet medical journal on Monday, two dozen prominent international scientists backed a recent call from G7 nations for further inquiry into the origins of the virus that causes Covid-19, and urged the World Health Organization to "expeditiously" continue a study with experts in China and the Chinese government.
"It is time to turn down the heat of the rhetoric and turn up the light of scientific inquiry," said the scientists, who were also among signatories of a February 2020 letter in the same journal "condemning conspiracy theories suggesting that Covid-19 does not have a natural origin".
Such theories - particularly that the virus could have leaked from a Chinese laboratory - have gained traction in recent months and a growing chorus of scientists, as well as the US government, are now calling for a deeper examination of the possibility.
Critics say the lab leak theory was dismissed too quickly, including in the earlier Lancet letter published less than two months after the world became aware of the first Covid-19 cases in China.
In the latest letter, the scientists - including Jeremy Farrar, head of the London-based Wellcome charitable foundation, Dennis Carroll, former director of the USAID's emerging pandemic threats unit, and leading German virologist Christian Drosten of the Charite university hospital in Berlin - stand by their original view that the virus emerged in nature.
"We believe the strongest clue from new, credible, and peer-reviewed evidence in the scientific literature, is that the virus evolved in nature, while suggestions of a laboratory-leak source of the pandemic remain without scientifically validated evidence that directly supports it in peer-reviewed scientific journals," the researchers wrote.
"Allegations and conjecture are of no help, as they do not facilitate access to information and objective assessment ... Recrimination has not, and will not, encourage international cooperation and collaboration," they said.
However, they acknowledged that their views were "neither data nor conclusions" and that inquiry should be guided by the scientific process, including "ongoing dialogue" and asking "new questions".
"Careful and transparent collection of scientific information is essential to understand how the virus has spread and to develop strategies to mitigate the ongoing impact of Covid-19, whether it occurred wholly within nature or might somehow have reached the community via an alternative route."
It comes as questions remain over how international efforts to uncover the origins of the virus will move forward after the controversial conclusion of a four-week "phase one" WHO-backed mission to China earlier this year.
The subject was on the table at the Group of Seven summit last month, with leaders calling for "a timely, transparent, expert-led, and science-based" phase-two mission led by the WHO, including in China. Beijing has also voiced support for further origins research, but says the "China part" of the investigation has been done and work needs to be conducted elsewhere.
Leo Poon Lit-man, a professor in the University of Hong Kong's School of Public Health who signed both Lancet letters, said he and other scientists were concerned about the uncertainty over the phase-two study.
"It's hard for science to move on when there is a lack of dialogue and communication," said Poon, pointing to arguments over different theories as a stumbling block.
"We just don't have a healthy dialogue and, in terms of public health, we have to build on trust ... I just don't see there is trust between governments and within scientific communities, and we need to find a way to reconcile this issue," he said, adding that an open mind was needed but it made sense to explore "more probable" scenarios first.
Experts on the WHO-backed mission to China said the lab leak theory - often linked to the Wuhan Institute of Virology - was the least likely of four potential scenarios for how the virus, thought to have come from a bat, passed to humans. They recommended further research in China and other regions.
But after their report was released in March, WHO chief Tedros Adhanom Ghebreyesus said all hypotheses remained on the table and called for a more robust assessment of the lab leak theory. He also noted issues in accessing raw data during the China visit and called for "more timely and comprehensive data sharing".
There has since been a surge in interest in the lab leak theory, with a separate group of scientists, in a May letter to Science, calling for both natural origin and lab leak theories to be taken seriously "until we have sufficient data".
US President Joe Biden later that month set a 90-day deadline for US intelligence agencies to assess the two hypotheses.
China has fiercely rejected the idea of a lab leak, defended its transparency and accused "certain countries" of "bullying and coercion" of scientists to support the theory.
"It's common understanding that the origin-tracing of Covid-19 must not be politicised and that it must be based on science, conducted through cooperation and ... with global collaboration," China's foreign ministry said in a statement on Friday.
Several signatories of the July 5 letter are also members of a Lancet Covid-19 Commission task force focused on the origins of the virus, while one of them, EcoHealth Alliance president Peter Daszak, was part of the WHO mission to China. Critics have said that Daszak - who has collaborated with the Wuhan Institute of Virology on US government-funded research via his non-profit research organisation - had a conflict of interest during his work on the WHO mission and in publishing the first Lancet letter last year. Daszak disclosed details of his work in China in an addendum to the February letter that was released last month and also included in the latest letter.
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