China is investigating the origins of Covid-19 in tests on tens of thousands of blood samples from the city of Wuhan, where the virus was first identified, a Chinese official has said.
Up to 200,000 samples were highlighted by the World Health Organisation as a potential clue to when and where the coronavirus first crossed into humans.
It comes as US President Joe Biden calls for transparency over the emergence of the virus.
The samples in the Wuhan Blood Centre are thought to span 2019, providing real-time tissue samples from many of the population in the central Chinese city where SARS-CoV-2 is thought to have first infected humans.
Chinese officials said blood bank samples are retained for two years in case they are needed as evidence in any lawsuits related to blood donations they came from.
That two-year waiting period will soon expire for the key months of October and November 2019, when most experts think the virus could first have infected humans.
Preparation for testing is now under way and set to begin once the two-year limit has been reached, an official from China's National Health Commission told CNN.
“This provides the closest in the world we've seen of real-time samples to help us understand the timing of the outbreak event,” said Huang Yanzhong, senior fellow for global health at the Council on Foreign Relations think tank.
They “absolutely will contain vital clues”, said Maureen Miller, associate professor of epidemiology at Columbia University.
She urged China to permit foreign experts to observe the process.
“No one will believe any results that China reports unless there are qualified observers at the very least,” she said.
The head of the Chinese team working on the WHO investigation, Liang Wannian, first said in July that China would test the samples.
He said once Chinese experts “have the results, they will deliver them to both the Chinese and foreign expert teams".
He said the samples came from the opening tube of a donor blood pouch, sealed shut and then stored, and Chinese experts had “made several assessments and evaluations on the testing methods and action plan, which will be implemented after the expiry” of the two-year limit.
The samples, if stored correctly, could contain crucial signs of the first antibodies made by humans against the disease, experts say.
Mr Liang said in July that while the first reported case was in Wuhan on December 8, 2019, “our research and the previous related research papers of Chinese scientists fully suggest ... December 8 is probably not the primary case.
“There might be other cases that occurred before.”
The Biden administration conducted a 90-day review of the intelligence over how the virus originated.
An unclassified report had officials still considering both natural transmission from animal to humans and a lab leak as plausible theories – unable to determine which was the more likely.
Mr Biden said: “Critical information about the origins of this pandemic exists in the People's Republic of China, yet from the beginning, government officials in China have worked to prevent international investigators and members of the global public health community from accessing it.”