Extremely dangerous virus mutation
The new coronavirus is able to quickly mutate and multiple strains could account for differing impacts around the world.
Research by China's top scientists found new strains of the virus not previously reported.
Professor Li Lanjuan and her colleagues from Zhejiang University also found direct evidence that certain mutations could create strains deadlier than others.
The team took viral strains from 11 randomly chosen COVID-19 patients from Hangzhou and tested how efficiently they could infect and kill cells, the SCMP reports.
The researchers observed "diverse mutations" among the virus strains including one that generates 270 times as much viral load as the least potent type.
Strains of the virus that create high viral loads are more dangerous.
The deadliest strains have been detected in Europe and the US, which is one of the world's worst-hit countries.
The weaker mutations still pose a threat to humans, with at least two Chinese patients becoming extremely unwell. One needed treatment in intensive care.
The study, published on Medrvix, showed "mutations can have a direct impact on the viral load".
The researchers "observed abundant mutational diversity, including several founding mutations for different major clusters of viruses now circulating globally".
Generic researchers have noticed differences between strains in different geographic areas.
The mystery of varying mortality rates was complicated by factors such as age, health conditions and a patient's blood type.
The findings mean the true diversity of the viral strains may have been largely underestimated.
Hospitals generally treat COVID-19 patients as though they have one disease, but the mutations mean a different response might be needed.
The study has implications for drug and vaccine development, which will have to take the virus mutations into account.
The sudden outbreak of the severe acute respiratory syndrome coronavirus (SARS-CoV-2) - which causes COVID-19 - has infected more than 2.4 million people worldwide
Nearly 170,000 deaths have been recorded since the outbreak began, which was first detected in Wuhan, China.
COVID-19 can be transmitted by asymptomatic patients, who show no fever, gastrointestinal or respiratory symptoms, making it much more challenging to prevent the spread.
It can also remain infectious in aerosols for multiple hours and up to seven days on surfaces.
So far no effective vaccines or cures have been found.
Originally published as Extremely dangerous virus mutation