Scientists in China studying the outbreak of disease caused by the coronavirus say they have found that two main strains of the virus are circulating in humans and causing infections.

The researchers, from Peking University’s School of Life Sciences and the Institut Pasteur of Shanghai under the Chinese Academy of Sciences, cautioned that their study looked only at a limited range of data, and said follow-up studies of larger data sets were needed to better understand the virus’s evolution.

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The preliminary study found that a more aggressive strain of the new coronavirus associated with the outbreak in Wuhan accounted for about 70 per cent of analysed cases while 30 per cent were linked to a less aggressive type.

The prevalence of the more aggressive virus type decreased after early January 2020, they said.

“These findings strongly support an urgent need for further immediate, comprehensive studies that combine genomic data, epidemiological data, and chart records of the clinical symptoms of patients with coronavirus disease 2019 (COVID-19),” they wrote in a study published on Tuesday in the National Science Review, the journal of the Chinese Academy of Sciences.

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Experts not directly involved in the study said its findings were interesting, but cautioned against drawing firm conclusions from such preliminary research.

“It’s difficult to confirm studies like this without a direct side-by-side comparison of pathogenicity and spread in, ideally, an animal model, or at least a greatly extended epidemiological study,” said Stephen Griffin, a professor and expert in infection and immunity at Britain’s Leeds University.

Also on Wednesday, one of China’s top medical associations said that the median incubation period of the coronavirus is five to seven days and the maximum 14 days.

The remarks by Du Bin, chairman of the critical care medical branch of the Chinese Medical Association, mark the most conclusive assessment of the virus’ incubation period by a government-affiliated medical organisation to date.