Scientists in China have discovered more than 30 mutations of the new coronavirus, which they say may partly explain why it has been more deadly in certain parts of the world.
Researchers from Zhejiang University said they have "direct evidence" that the virus "has acquired mutations capable of substantially changing its pathogenicity".
The study was written by a team including Professor Li Lanjuan, one of China's top scientists who was reportedly the first expert to propose a lockdown in Wuhan - where COVID-19 originated.
Samples were taken from 11 patients admitted to hospitals in Hangzhou, 470 miles east of Wuhan, between 22 January and 4 February during the early phase of the outbreak.
Using "ultra-deep sequencing", researchers identified 33 mutations of the coronavirus - known as SARS-CoV-2 - of which 19 were new.
The deadliest mutations in the patients in the study had also been found in most patients across Europe, the South China Morning Post reported.
Meanwhile, the milder strains were the predominant types found in parts of the United States, such as Washington state, the newspaper said.
One mutation found in five patients involved in the research had previously only been seen in one case in Australia, according to the study.
The researchers said the findings indicate "the true diversity of the viral strains is still largely underappreciated".
They also warned vaccine developers need to consider the impact of these "accumulating mutations... to avoid potential pitfalls".
In the study, the researchers assessed the viral load - meaning the amount of the virus - in human cells after one, two, four and eight hours, as well as the following day and 48 hours later.
The most aggressive strains created up to 270 times as much viral load as the least potent type, the scientists found.
Prof Li and her colleagues said their findings also indicated that a "higher viral load leads to a higher cell death ratio".
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Ten of the 11 patients involved in the study - which included eight males and three females aged between four months and 71 years old - had "moderate or worse symptoms" of COVID-19.
They have all since recovered from the virus, according to the research which has been published by medRxiv.
The findings, which have not been peer-reviewed, were approved by Zhejiang University, the study said.
There have been more than 2.4 million coronavirus cases around the world and more than 170,000 deaths, according to Johns Hopkins University, which is tracking the outbreak.