There are fears of flu pandemic

In preliminary tests, it was found to protect animals against various strains of the virus - and may also protect against future pandemic strains.

University of Warwick researchers used a flu virus naturally stripped of some genetic material to compete with other invading flu viruses.

This slowed the rate of infection so much the body could fight it off.

In effect, the invading virus became its own vaccine by triggering an immune response sufficiently powerful to neutralise it before it could gain a strong enough foothold.

The Warwick team plan to develop the treatment as a nasal spray.

Experts warned much more testing was required.

This is cutting edge science, but there is a lot that could still go wrong Professor John Oxford

However, they said the development of the vaccine was timely, amid concerns the H5N1 bird flu strain circulating in south east Asia could mutate into a pandemic strain which would put millions of lives at risk.

Existing vaccination methods depend on stimulating the body's immune system, so that white blood cells produce antibodies that attach to the surface of the virus and start the process of killing it.

This works well for many diseases, such as smallpox, polio and measles, but is much less effective with flu, as the coat of the flu virus is continually changing.

Vaccination against one strain of flu is totally ineffective against another.

Crowded out

Professor Nigel Dimmock has spent more than two decades developing the new approach.

The "protecting virus" with which he worked had naturally lost around 80% of the genetic material of one of its eight RNA constituent segments. This deletion makes the virus harmless and prevents it from reproducing by itself within a cell, so that it cannot spread like a normal influenza virus.

However, if it is joined in the cell by another influenza virus, it retains its harmless nature but starts to reproduce - and at a much faster rate than the new influenza virus.

This fast reproduction rate - spurred by the new flu infection - means that the new invading influenza is effectively crowded out.

This vastly slows the progress of the new infection, prevents flu symptoms, and gives the body time to develop an immune response to the harmful new invader.

One size fits all

The Warwick team believes its research indicates the protecting virus would have the same effect regardless of the strain of flu infection.

This is because the coat of the virus is irrelevant to the protection process - the effect works on the virus genes inside the cell.

In addition it protects instantly, whereas protection generated by conventional flu vaccination takes two to three weeks to become fully effective.

Experiments so far show that a single dose of protecting virus can be given six weeks before, and 24 hours after an infection with flu virus and be effective.

The Warwick research team has now filed a patent on the protecting virus and is exploring ways of taking it through human clinical trials and testing on birds.

Professor John Oxford, a virologist at Queen Mary College School of Medicine, London, said: "This is cutting edge science, but there is a lot that could still go wrong.