Calls for an independent investigation into the origin of the coronavirus, which devastated the world after emerging from Wuhan have become louder especially in the US following new reports that some of the researchers at the Wuhan Institute of Virology (WIV) fell sick well before China officially announced the COVID-19 outbreak on December 30, 2019.

Answering questions on calls for investigations, Chinese Foreign Ministry spokesman Zhao Lijian referred to the study of the origin of the coronavirus led by WHO experts group which visited Wuhan and WIV from January 14 to February 10, but parried a question whether Beijing would agree for an independent probe to investigate the allegations that COVID-19 may have been leaked from the WIV.

Experts of the WHO mission during their visit to Wuhan had field visits and went through a lot of data and released a joint study report with many conclusions, Zhao said.

On Tuesday, media reports from Washington quoted the White House coronavirus adviser Andy Slavitt as saying that "we need to get to the bottom" of the origins of the pandemic pathogen and the World Health Organization (WHO) and China need to do more to provide definitive answers for the global community.

"We need a completely transparent process from China," Slavitt told a coronavirus task force briefing. Full assistance from the WHO is needed, and "we don''t have that now."

The Wall Street Journal has reported on May 24 that six miners fell sick with a mysterious illness akin to coronavirus in April 2012 after entering the mine located in the outskirts of a village deep in the mountains of southwest China which was investigated by the top researchers of the WIV.

Researchers from WIV now claim that the virus is closer to pangolins and “unlikely from Wuhan lab".

Latest research again indicates that it was unlikely the novel coronavirus was leaked from the WIV, state-run Global Times quoted experts as saying, amid new reports about the lab leak theory ahead of the meeting of the World Health Assembly (WHA) the governing body of the WHO, which began its sessions from Monday.

Researchers from the WIV and the University of Chinese Academy of Sciences, including Shi Zhengli, who has been dubbed China''s "Bat Woman" for her research with bats and viruses, published a report last Friday on BioRxiv that further refutes the hyped theory that the virus came from the laboratory, the report said.

Bats and pangolins are recognised as the most probable reservoir hosts that harbour viruses which are very similar to SARS-CoV-2.

“Based on the Friday report, it is safe to say that bats are probable ancestors of the coronavirus that led to SARS in 2003 and the recent COVID-19 pandemic, and coronavirus strains discovered in pangolins are closer to the novel coronavirus in humans," Yang Zhanqiu, a virologist from the Wuhan University, told the Global Times.

He, however, said the report still did not explain how the virus was transferred and adapted from bats to humans via pangolins.

The most likely process is that the coronavirus from bats had mutated in nature for decades before it successfully infected humans and led to the deadly COVID-19 pandemic, Zhuang said.

The news reports by the WIV researchers came after two reports by the WSJ stating that three researchers from the WIV sought hospital care in November 2019, weeks before Beijing disclosed the COVID-19 pandemic, which China has denied.

WSJ, in another report on May 24 said WIV scientists in April 2012 have investigated the illness suffered by six miners who fell sick with a mysterious health issue after entering the mine to clear bat guano. Three of them died.

China has currently blocked the international media from reaching the mine to see the surroundings, the report said.

The WSJ quoted master’s thesis by Li Xu from the No. 1 School of Clinical Medicine at Kunming Medical University in southwest China on the miners'' illness akin to COVID-19.

Over the next week, five others working at the Mojiang mine, ages 30 to 63, were admitted to the same hospital. All had similar symptoms.

Zhong Nanshan, top pulmonologist of China has diagnosed the illness as pneumonia, most likely caused by a virus, and recommended testing for SARS antibodies and trying to identify the type of bats in the mine, the WSJ report said.

It quoted another thesis, written by a Ph.D. candidate supervised by George Gao, the current head of the Chinese Centre for Disease Control and Prevention, or China CDC, said four of the miners tested positive for SARS antibodies.

Over the next year or so, WIV scientists entered the Mojiang mine and took faecal samples from 276 bats, identifying six different species, according to a research paper they published later, it said.

They extracted genetic material from the samples and sequenced fragments. Half of the samples tested positive for coronaviruses, including an unidentified SARS-like strain, according to the scientists. They called the virus RaBtCoV/4991, it said.

Critically, all six bat species showed evidence of coronavirus co-infection, the researchers found.

In other words, the virus could easily exchange genetic material with similar ones to create a new coronavirus— an environment ripe for the creation of new viruses that could potentially infect humans, the WSJ report said.

That research was led by Shi Zhengli, the WIV’s leading bat coronavirus expert. When the results were published in 2016 in the journal Virologica Sinica, it said.

“Only after the COVID-19 pandemic began did it become more significant. In February 2020, Dr. Shi and her colleagues published a paper in the scientific journal, Nature, revealing the existence of a virus called RaTG13. Sequencing had revealed it was 96.2 per cent similar to SARS-CoV-2 genetically, making it the closest known relative to the pandemic virus,” the WSJ report said.

“They said it was found in a bat in Yunnan, the Chinese province that includes the Mojiang region mine, but didn’t say when or where,” it said.

“That revelation was considered a breakthrough in the search for COVID-19’s source, strongly indicating that it originated in bats,” it said.