Top US infectious disease expert Dr Anthony Fauci has urged China to release information about six labourers who fell ill after working in a mine in Yunnan province in 2012, and are now seen as a key part of efforts to find the origins of Covid-19.

The workers, ages 30 to 63, were scrubbing a copper seam clean of bat faeces in April 2012. Weeks later, they were admitted to a hospital in the provincial capital of Kunming with persistent coughs, fevers, head and chest pains and breathing difficulties. Three eventually died.

The mine is in Mojiang in southwest China, about 1,500 kilometres from Wuhan, where Covid-19 was first identified.

What do we know about the six mine workers?

Though the full biographical details of the six workers have not been released, their surnames, ages and medical records were published in a 2013 thesis written by a Kunming Medical University postgraduate student named Li Xu.

Li's study, still available on China's scientific paper archive at cnki.net, examines each patient's symptoms and concludes they were victims of a "SARS-like" coronavirus contracted from horseshoe bats.

Scientists returning to the mine at the end of 2012 found samples of a pathogen that came to be known as the "Mojiang virus", found in rats and unrelated to SARS-CoV-2.

Subsequent research was unable to confirm whether it caused the miners' illness.

According to the Wuhan Institute of Virology's (WIV) Shi Zhengli, China's top bat coronavirus researcher, the workers' pneumonia-like symptoms were caused by a fungal infection.

Shi and her team also said in research published last November that they had retested 13 serum samples from four of the patients and found no sign they had been infected with SARS-CoV-2.

Why are the cases in the public eye?

Since the middle of last year, Li's postgraduate thesis has been circulated online as purported evidence that a coronavirus very similar to SARS-CoV-2 could have been infecting humans as early as 2012.

Some also believe the paper provides circumstantial evidence for broader allegations that WIV had captured, studied and conducted "gain of function" experiments on viruses found in the mine, including RaTG13.

First identified in 2016, RaTG13 shares 96.2 per cent of its genome with SARS-CoV-2, according to a paper released by Shi and other researchers early in February 2020, just weeks after the first Covid-19 cases had been identified in Wuhan.

What other viruses were found in the mine?

From 2012 to 2015, WIV researchers identified as many as 293 coronaviruses in and around the mine.

The institute in November 2020 disclosed the existence of eight other "SARS-type" coronavirus samples taken from the site.

In a preprint last month, Shi and other researchers said none of the eight was a closer match to SARS-CoV-2 than RaTG13. Crucially, none of them possessed the key receptor binding domain that allows SARS-CoV-2 to infect humans so efficiently.

The paper concluded that “the experimental evidence cannot support” claims that SARS-CoV-2 was leaked from the lab, and called for “more systematic and longitudinal sampling of bats, pangolins or other possible intermediate animals” to better understand where the pandemic originated.