The global outbreak that has sickened nearly 100,000 people across six continents may be fuelled by two variants of the same coronavirus: one older and less aggressive and a newer version whose mutations may have made it more contagious and more deadly, according to a controversial new study.

Chinese scientists who compared the genetic sequences of 103 viral samples from patients infected with COVID-19 said their evidence suggests that the virulent version of the coronavirus - which they tagged the "L-type" version - was the dominant strain in the earliest phase of the outbreak that began in Wuhan, China, late last year. That strain, they said, appeared to recede as the epidemic progressed.

An illustration of the 2019 novel coronavirus provided by the US Centres for Disease Control and Prevention. Credit:AP

But among samples collected later, as COVID-19 spread across China and into other countries, a variant of the virus they dubbed the "S-type" was more common, the scientists reported. They suggested that the genetic make-up of the S version more closely resembles coronaviruses circulating in bats and pangolins, the animals that are thought to have incubated the virus before it jumped to humans. And they surmised that it is a less virulent version.

Their findings suggest the S-type version of the coronavirus may have escaped its animal hosts earlier than previously believed - and that it may have been circulating longer without causing enough illness to set off alarm bells.