The natural origins theory of the novel coronavirus is still "the most likely," White House Chief Medical Advisor Anthony Fauci said recently. 

"The most likely explanation is a natural evolution from an animal reservoir to a human," Fauci, also the director of the National Institute of Allergy and Infectious Diseases of the US, told the media Saturday.

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Paul Offit, a member of the US Food and Drug Administration's vaccine advisory committee, echoed Fauci's remarks. "I think the chance that this was created by laboratory workers – that it was engineered – is zero."

Meanwhile, the head of the World Health Organization (WHO) recently said it was "premature" to rule out a potential link between the Covid-19 pandemic and a laboratory leak.

WHO Director-General Tedros Adhanom Ghebreyesus Thursday said getting access to raw data had been a challenge for the international team that travelled to China earlier this year to investigate the source of Covid-19.

He said there had been a "premature push" to rule out the theory that the virus might have escaped from a Chinese government lab in Wuhan city – undermining the WHO' March report, which concluded that a laboratory leak was "extremely unlikely."

The first human cases were identified in Wuhan. Tedros told reporters that the UN health agency is "asking China to be transparent, open and cooperate, especially on the information, raw data that we asked for at the early days of the pandemic."

"I was a lab technician myself, I'm an immunologist, and I have worked in the lab, and lab accidents happen. It's common," he said.

In recent months, the idea that the pandemic started somehow in a laboratory – and perhaps involved an engineered virus – has gained traction, especially with President Joe Biden ordering a review of US intelligence to assess the possibility in May.

China has struck back aggressively, arguing that attempts to link the origins of Covid-19 to a lab are politically motivated and has suggested that the outbreak might have started abroad.

Most scientists suspect that the coronavirus originated in bats but the exact route by which it first jumped into people – via an intermediary animal or in some other way – has not yet been determined. It typically takes decades to narrow down the natural source of an animal virus like Ebola or SARS.

"We need information, direct information on what the situation of this lab was before and at the start of the pandemic," the WHO chief said, adding that China's cooperation was critical. "If we get full information, we can exclude (the lab connection)."

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Numerous public health experts have also called for an independent examination of Covid-19's origins, arguing the WHO does not have the political clout to conduct such a forensic analysis, and that the UN agency has failed after more than a year to extract critical details from China.

Jamie Metzl, who has led a group of scientists calling for a broader origins investigation, welcomed Tedros' comments but said it was "deeply unfortunate and dangerous" that there were no current plans for a probe led by experts beyond the UN health agency, saying that China has repeatedly blocked requests for all relevant records and samples.

Georgetown University law professor Lawrence Gostin, an expert in public health law, said Tedros' unusual plea for Chinese cooperation underlines how weak the WHO is. "It has no power or political heft to demand access to information critical for global health."

Any WHO-led mission to China also requires government approval for all experts who travel to the country, as well as permission to visit field sites and final approval on any trip report.

Tedros' appeal for transparency was echoed by German Health Minister Jens Spahn, who urged Chinese officials to allow the investigation into the origins of the virus to proceed.

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"We do appreciate the cooperation of the Chinese government so far for the first mission. But that's not yet enough," he said.