CHICKEN flu poses a "massively frightening" potential threat to humanity, a leading medical journal says today.

At least five people in Vietnam are now known to have died in the latest outbreak of dangerous influenza carried by birds.

The Lancet medical journal warned in an editorial that standard vaccines would be useless against the virus if it started spreading through human communities.

At present the avian influenza strain, known as H5N1, is not thought capable of passing from person to person.

But experts fear it might acquire the ability if its DNA mingles with that of a human flu virus. Such "reassortment" is a well-known feature of viruses.

If this were to happen, the scene could be set for a major world-scale disaster like the Spanish flu pandemic of 1918, which killed up to 40 million people.

The Lancet said, "The fear is that a strain such as H5N1 might reassort with a human influenza virus to become contagious among people. In view of the high mortality of human influenza associated with this strain, the prospect of a worldwide pandemic is massively frightening."

Wild birds were the main source of avian influenza, but live poultry markets in Asia provided the virus with a breeding ground.

In 1997, 18 people in Hong Kong were admitted to hospital with H5N1 infection, and six died.

This outbreak was the first documented example of direct transmission of the strain to humans.

It resulted in 1.4 million chickens and other poultry birds - the entire stock in Hong Kong - being destroyed.

Another outbreak in the former colony in 1999 saw 1.25 million birds culled.

Last year two Hong Kong residents developed H5N1 influenza after returning from China, and one died. In the same year, a related strain, H7N7, broke out in the Netherlands poultry industry and killed a vet.

The latest outbreak has sent experts from the World Health Organisation, the US Centres for Disease Control and Prevention, and the UN Food and Agriculture Organisation, racing to Vietnam.

By Tuesday, a fifth person in the country had died from H5N1 influenza, according to the WHO. Bird stocks in Japan and South Korea had also been infected.

Traditional vaccines cannot be made to combat either H5 or H7 avian influenza. The viruses rapidly kill off the chick embryos normally used in the vaccine production process.

Genetic engineering can also be employed, but vaccines made by this method have not yet been studied in clinical trials, said The Lancet.

Antiviral drugs, meanwhile, are expensive and not effective enough against the viruses.

The Lancet added that influenza was more contagious than Severe Acute Respiratory Syndrome. It was unlikely to be controlled by quarantine measures effective against Sars.