The number of people already infected by the mystery virus emerging in China is far greater than official figures suggest, scientists have told the BBC.

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There have been 49 laboratory-confirmed cases of the new virus, but UK experts estimate the figure is closer to 1700.

In New Zealand, the Director of Public Health Dr Caroline McElnay said the health ministry had been in frequent communication with the sector to give advice, since becoming aware of the novel coronavirus.

Two people are known to have died from the virus, which appeared in Wuhan city in December.

"I am substantially more concerned than I was a week ago," disease outbreak scientist Prof Neil Ferguson said.

The work was conducted by the MRC Centre for Global Infectious Disease Analysis at Imperial College London, which advises bodies including the UK government and the World Health Organization.

Singapore and Hong Kong have been screening air passengers from Wuhan and US authorities announced similar measures starting on Friday at three major airports in San Francisco, Los Angeles and New York.

There are no plans to introduce thermal screening at airports in New Zealand.

The Ministry of Health said research had shown thermal screening to be generally ineffective at detecting influenza.

"A 2014 Canadian review concluded that the high number of false detections along with the limited effectiveness meant that the evidence did not support introducing them."

How were the numbers calculated?

The crucial clue to the scale of the problem lies in the cases being detected in other countries.

While the outbreak is centred on Wuhan, there have been two cases in Thailand and one in Japan.

"That caused me to worry," Prof Ferguson said.

He added: "For Wuhan to have exported three cases to other countries would imply there would have to be many more cases than have been reported."

It is impossible to get the precise number, but outbreak modelling, which is based on the virus, the local population and flight data, can give an idea.

Wuhan International Airport serves a population of 19 million people, but only 3400 a day travel internationally.

The detailed calculations, which have been posted online ahead of publication in a scientific journal, came up with a figure of 1700 cases.

What does it all mean?

Prof Ferguson said it was "too early to be alarmist" but he was "substantially more concerned" than a week ago.

Chinese officials say there have been no cases of the virus spreading from one person to another.

Instead, they say the virus has crossed the species barrier and come from infected animals at a seafood and wildlife market in Wuhan.

Prof Ferguson argues: "People should be considering the possibility of substantial human-to-human transmission more seriously than they have so far.

"It would be unlikely in my mind, given what we know about coronaviruses, to have animal exposure be the principal cause of such a number of human infections."

Understanding how a novel virus is spreading is a crucial part of assessing its threat.

What is this virus?

Viral samples have been taken from patients and analysed in the laboratory.

And officials in China and the World Health Organization have concluded the infection is a coronavirus.

Coronaviruses are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.

At the mild end, they cause the common cold, but severe acute respiratory syndrome (Sars) is a coronavirus that killed 774 of the 8098 people infected in an outbreak that started in China in 2002.

Analysis of the genetic code of the new virus shows it is more closely related to Sars than any other human coronavirus.

The virus has caused pneumonia in some patients and been fatal in two of them.

What do other experts say?

Dr Jeremy Farrar, director of the Wellcome medical research charity, said: "There is more to come from this epidemic.

"Uncertainty and gaps remain, but it's clear that there is some level of person-to-person transmission.

"We are starting to hear of more cases in China and other countries and it is likely, as this modelling shows, that there will be many more cases, in a number of countries."

Prof Jonathan Ball, from the University of Nottingham, said: "What's really important is until there has been widespread laboratory testing it is very difficult to put a real number on the cases out there.

"But this is a figure we should take seriously until we know otherwise ... [49] ... animal-to-human 'spillovers' is stretching it a bit and there probably is more underlying infection than has been detected so far."

Earlier this week, University of Otago biochemistry Professor Kurt Krause said it was important to note that coronaviruses were not uncommon.

However, he said there was a lot more work to be done to find out who was susceptible, how it's transmitted - and with what incubation period and severity, as well as control measures and ways to find possible animal reservoirs of infection.

"It is an important story that deserves a close watch over the next weeks and months," he said.

He said one concerning feature was the close genetic relationship of the novel virus to the deadly Sars variant.

That meant New Zealand authorities should still be on the lookout.

"I think we want to certainly be careful about people who have travelled, who have visited that market, following contacts - I understand [the WHO] is following about 700 people who have had contact with the people who have had infections - do they have any symptoms," he said.

- RNZ / BBC