Paris (AFP) - Scientists have taken a major step towards creating a vaccine that works against multiple strains of influenza, according to two studies published Monday in top journals.

A "universal vaccine" is the holy grail of immunisation efforts against the flu, a shape-shifting virus which kills up to half a million people each year, according the World Health Organization.

There have been several killer pandemics in the last century -- the 1918 Spanish Flu outbreak claimed at least 20 million lives.

Existing vaccines target a part of the virus that mutates constantly, forcing drug makers and health officials to concoct new anti-flu cocktails every year.

In the two studies, published in Nature and Science, researchers tested new vaccines on mice, ferrets and monkeys that duplicate another, more stable, part of the virus.

Scientists have long known that the stem of haemagglutinin -- a spike-like protein, known as HA, on the surface of the virus -- remains largely the same even when the tip, or "head", changes.

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Existing flu vaccines target a part of the virus that mutates constantly, forcing drug makers and he …

But until now, they have not been able to use the stem to provoke an immune reaction in lab animals or humans that would either neutralise the virus, or allow the body to attack and destroy infected cells.

To make that happen, a team led by Hadi Yassine of the Vaccine Research Center at the US National Institutes of Health grafted a nano-particle-sized protein called ferritin onto a headless HA stem.