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

A man wearing a face mask rides a kick scooter through an intersection in Wuhan, the epicentre of the novel coronavirus outbreak, Hubei province, China March 3, 2020. REUTERS/Stringer

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.

The preliminary study found that a more aggressive strain of the new coronavirus associated with the outbreak in Wuhan accounted for about 70% of analyzed cases, while 30% 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.

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 organization to date.

The revelations came amid a fall in new coronavirus cases following crippling restrictions imposed on the world’s second largest economy to stop its spread, including transport suspensions and the extension of the Lunar New Year holiday.

NEW CASES DOWN

Mainland China had 119 new confirmed cases of coronavirus on Tuesday, the National Health Commission said, down from 125 the previous day, in a broad trend that has seen numbers of new cases fall from the middle of February.

The total number of cases on the mainland has now reached 80,270. The number of deaths rose by 38 to bring the total toll for mainland China to 2,981 by March 3.

All but one new death occurred in Hubei province, the epicenter of the outbreak.

With the number of new daily infections overseas now exceeding new cases in China, Chinese officials have begun to seek ways to control the spread of the virus outside of China and guard against future outbreaks.

Authorities have asked overseas Chinese hoping to return home to reconsider their travel plans, while cities across the country have set up quarantine rules for those entering from high-risk places.

An infected person is known to have arrived in China from Iran, one of the virus’ new hotspots, last week.

China is encouraging domestic producers of medical protective equipment to export protective suits to meet overseas demand as the virus spreads, Cao Xuejun, an official with China’s Ministry of Industry and Information Technology said in a press briefing in Beijing on Wednesday.

China’s health authorities are also studying setting up emergency reserves for medical resources and protective materials, Mao Qunan, an official at China’s National Health Commission said at the same briefing.