The Centre for Health Protection said that the student was found to have common viruses unrelated to the Wuhan cases, including a type of coronavirus, while the other five new cases were found to have flu or other kinds of known viruses. Seven people have already been discharged from hospital.

So far, test results show that none of the cases reported outside mainland China – including one recent case in Singapore – were related to the Wuhan virus.

While the laboratory results have dispelled fears of a return of the deadly Sars epidemic in 2003 that killed hundreds in mainland China and Hong Kong, the unconfirmed source and unidentified strain of a possible new form of pneumonia has caused alarm among health experts.

“There is limited information to determine the overall risk of this reported cluster of pneumonia of unknown [cause],” the World Health Organisation’s China office said on Sunday.

World Health Organisation ‘closely monitoring’ China’s viral pneumonia outbreak

“The reported link to a wholesale fish and live animal market could indicate a link to exposure to animals.”

The WHO added that it would continue to monitor the situation closely.

The first cases of the mysterious virus were reported last week and many of those affected worked at Wuhan’s Huanan seafood market, which has since been closed.

However, the number of those in a “severe condition” has now fallen from 11 to seven.

A total of 163 people who had been in close contact with the infected patients have been placed under medical observation. No deaths have been reported.

“Preliminary investigations have not shown evidence of human-to-human transmission of the virus. But the work to identify the virus is still ongoing, as we are still uncertain of the source and cause of the virus,” said Li Gang, director of the Wuhan Centre for Disease Prevention and Control, told the local newspaper Changjiang Ribao .

Professor David Hui Shu-cheong, a specialist in respiratory medicine at Chinese University of Hong Kong, said that based on the available information available, the virus was likely to be a type of coronavirus.

China says Wuhan pneumonia is not Sars

This virus family includes the common cold and Sars, and the virus is likely to be contagious among human, he said.

“It is common that these new types of coronavirus are contagious from person to person. The difference is whether or not it is highly contagious,” Hui said, adding that the virus was clearly a new strain of pneumonia, since all typical diagnostic tests that have been carried out have been negative.

Hui said one common method to find out whether a virus was contagious among humans was to determine whether relatives of those infected had fallen ill despite not visiting the market at the centre of the outbreak themselves.

Another indication would be whether the medical personnel who have been in contact with patients have been infected.

If so, tests would then be performed to see whether the virus genomes show an exact match, which would in turn reveal whether the virus is contagious. However, Hui said it was a positive sign that the number of severe cases had fallen to seven.

“This at least shows that some people are able to recover from it,” he said.

Ho Pak-leung, a microbiologist at the University of Hong Kong, urged the mainland Chinese authorities to release more information – including the age, gender, suspected infected area and condition – of the patients in Wuhan.

China pneumonia outbreak raises spectre of Sars

“There is now more information from the mainland that shows that the virus could have first been transmitted from animals to humans,” Ho said.

“Although now the virus [has been confirmed not to be] Sars and Mers, [coronavirus] carried in animals has been identified as a common cause of pneumonia in humans in recent years,” Ho told Commercial Radio, noting that most cases identified have been linked to the wet market in Wuhan.

Ho added that in the previous Sars and Mers epidemics, around 20 per cent of those infected had passed the virus on to other humans.

Sars claimed the lives of 299 people in Hong Kong and 349 in mainland China in 2002 and 2003. Scientists later concluded that the virus originated from bats, and later passed through civets before reaching humans.