Doctors have revealed fresh details on the terrible toll taken on the body by Covid-19, releasing the results of autopsies of those who have died in the pandemic.

In a study published in The Lancet journal EClinicalMedicine, Dr Amy Rapkiewicz, the chair of the Department of Pathology at NYU Langone Medical Centre, showed the role played by blood clots in the progression of the disease.

Describing how scientists found clotting in tiny blood vessels throughout the body, Rapkiewicz told CNN the findings were "dramatic".

"Because though we might have expected it in the lungs, we found it in almost every organ that we looked at in our autopsy study."

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The autopsies also showed the extensive presence of megakaryocytes, large bone marrow cells that don't usually appear outside the lungs and bones.

"We found them in the heart and the kidneys and the liver and other organs," Rapkiewicz said. "Notably in the heart, megakaryocytes produce something called platelets that are intimately involved in blood clotting."

"I could not remember a case before where we saw that," Rapkiewicz told the Washington Post. "It was remarkable they were in the heart."

Speaking to TCTMD, Rapkiewicz said it is "a very interesting observation that seems to be consistent across multiple Covid cases."

Noting that Covid's effect on blood clotting is at the opposite end of the spectrum from other killer viruses such as ebola, Rapkiewicz said researchers need to be diligent and "learn from our history" and explore what is known about other contagious diseases that affect the body's coagulation systems.

The study comes as scientists continue their race to find vaccines and cures for the pandemic, which has killed more than 530,000 people across the globe.

People on six continents are already getting jabs in the arm as the race for a Covid-19 vaccine enters a defining summer, and even bigger studies are poised to prove if any shot really works - and maybe offer a reality check.

British and Chinese researchers are chasing the coronavirus beyond their borders, testing potential vaccines in Brazil and the United Arab Emirates because there are too few new infections at home to get clear answers.

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World Health Organization (WHO) Director-General, Dr Tedros Ghebreyesus, told reporters that the WHO is “looking into the origins of the virus.” Video / AP

The US is set to start the largest trials - 30,000 people to test a government-created shot starting this month, followed about a month later with another 30,000 expected to test a British one.

Those would likely be divided among Americans and volunteers in other countries such as Brazil or South Africa, Dr Anthony Fauci, of the National Institutes of Health, told AP.

While he's optimistic, "we've been burned before", Fauci cautioned.

-Additional reporting, AP