The vaccine is designed to protect against all flu strains

The vaccine, made by Acambis, should protect against all strains of influenza A - the cause of pandemics.

Currently, winter flu jabs have to be regularly redesigned because the flu virus keeps changing.

The new vaccine would overcome this and could be stockpiled in advance of a bird flu outbreak, say experts.

Promising results

Each year winter flu kills around 4,000 people in the UK.

Globally, between 500,000 and one million people die each year from influenza.

But a pandemic of the human form of bird flu, which experts believe is inevitable, could kill as many as 50m people worldwide.

The US trials show that the jab is safe and it works fast to make the body immune against flu.

It could be stockpiled in advance of a pandemic

Dr Michael Watson of Acambis

Nine out of 10 of those who had two doses of the jab ACAM-FLU-A developed antibodies against flu virus.

Scientists at Acambis are now working alongside Professor Walter Fiers and his team from the University of Ghent in Belgium and the Flemish Institute of Biotechnology to perfect the formulation before doing larger human trials.

Dr Michael Watson of Acambis said: "As a universal vaccine, ACAM-FLU-A can potentially overcome many of the drawbacks of existing influenza vaccines.

"It can be manufactured at any time of the year, and could be stockpiled in advance of a pandemic or potentially used routinely to ensure population protection against future pandemics."

Unique action

Current flu vaccines work by giving immunity to two proteins called haemagglutinin and neuraminidase, which are found on the surface of flu viruses.

However, these proteins keep mutating which means doctors have to keep making new vaccines to keep up.

The Acambis vaccine homes in on a different protein, called M2, which is found on the surface of all A-strains of flu and does not appear to mutate so readily.

Professor Ian Jones, a University of Reading virologist, said the jab could end the scramble to produce a new winter jab each year.

But he said it would still be some years before it was widely available for patients.

"Larger trials and tests on a wider range of viruses will be needed before the full potential for pandemic protection can be assured," he said.