Health experts in Hong Kong are stepping up their efforts to trace a new strain of flu previously found only in birds, in the hope of finding out more about the virus.

The move comes after the discovery of two more cases of the new flu, bringing the known total to four.

Hong Kong health officials have been holding urgent meetings after the discovery that two more humans have caught the strain that was thought to affect only birds.

The virus killed 4,500 chickens in the Hong Kong area earlier this year. Now the virus has moved on to humans, and scientists do not know how it could do that.

Influenza virus - the conventional strain

The development has sparked international concern because in the 1950s and 1960s similar viral shifts from birds to humans led to flu pandemics which affected millions of people worldwide.

Hong Kong health officials have asked the World Health Organisation to alert vaccine production centres worldwide with a view to preparing a vaccine if evidences emerges of further spreading of the virus.

A 54-year-old man, who died on Friday, and a 13-year-old girl in a critical condition are the two latest suspected cases.

If confirmed, it would bring the number of cases of the new H5N1 flu strain to four, with two deaths.

Two experts from the US Centers for Disease Control flew to Hong Kong to hold meetings with local officals.

On Sunday, Dr Paul Saw, the deputy director of health in Hong Kong and chairman of a special working group looking into the virus, said it was still unclear how the four people became infected with the virus.

"The virus does not appear to have human-to-human transmission. However we feel that this possibility would need to be further looked into," he said.

He stressed that while there was cause for concern, there was no need for panic as "the evidence does not support that the disease is widespread."

And experts writing in the British scientific journal Nature wrote in October that the strain must be closely monitored as it has "presently unknown pandemic potential".

The Standard newspaper quoted Daniel Lavanchy of the World Health Organisation headquarters in Geneva as saying the four victims had no contact with one another.

He called the virus "a new entity. These are the only four cases in the world and nobody knows the characteristics of the virus. This is a 50% death rate, which is extremely high," he said. "Major diseases usually kill less than 10% of the people."

When the disease first emerged earlier this year, killing a three-year-old boy and infecting a two-year-old, the Hong Kong government said the cases appeared to be isolated and posed no major threat to public health.

 
Tom Skinner of the US Centres for Disease Control on the virus (Dur 2'10")

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