A flu jab, to be given just once in a lifetime, can fight off all strains of flu, and should be available within the next five years, scientists say.

The vaccine would fight not only the winter flu that leaves millions feeling miserable each year, but also deadly pandemic viruses.

These findings were presented at a conference held by the Royal Society, London.

The current flu jab protects only three-quarters of those immunised and needs to be reformulated each year to keep on top of changes in the virus.

It targets a lollipop-shaped head of a protein that sticks out from the surface of flu particles. The protein is key to infection, but the head constantly mutates, meaning it differs from strain to strain, reports the Daily Mail.

The new drug, developed at the National Institute of Health (NIH) in Maryland, US, uses DNA to trick the body's immune system into producing antibodies against the 'lollipop stick'.

Unlike the head, the 'stick' changes little from strain to strain, which means one vaccine should protect against multiple strains of the virus.

Gary Nabel, NIH's director of vaccine research, used the DNA from the protein to trick animals' immune systems into producing antibodies that seek out and destroy the bug.

He then gave a 'booster shot' of a harmless flu or cold virus to ratchet up the immune response. The vaccine was also effective against bird flu.

Safety trials have begun on people and its effectiveness could be tested on patients by 2013. If the vaccine is as safe and powerful as the research team hopes, it should be in widespread use by 2015.