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.

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.

The next step was to immunise mice and ferrets, then injecting them with the H5N1 "bird flu" that has a mortality rate of more than 50 percent among people but is not very contagious.

- 'Exciting development' -

The mice were completely protected against the flu, the researchers found.

And most of the ferrets, the species that best predicts the success of influenza vaccines on humans, did not fall ill either.

Moreover, when a new batch of mice was injected with antibodies from the rodents which had survived the previous round, most of them also shook off what should have been a lethal dose of bird flu.

The other study, led by Antonietta Impagliazzo of the Crucell Vaccine Institute in Leiden, the Netherlands, took a similar approach -- also creating an HA "stem-only" vaccine.

It proved effective in mice. In monkeys, the vaccine provoked a high level of antibodies and significantly reduced fever following infection with the H1N1 virus, which is far less deadly than bird flu but highly contagious.

Other scientists not involved in the studies described them as a major step towards a universal vaccine, but cautioned that a lot of work has to be done, possibly over many years, before a vaccine can be tested on humans.

"This is an exciting development, but the new vaccines now need to be tested in clinical trials to see how well they work in humans," said Sara Gilbert of the University of Oxford, adding that it will take years to reach that point.

"This is an important proof of concept for this vaccine approach," David Morens, a senior advisor at the National Institute of Allergy and Infectious Diseases at the US National Institutes of Health, said of the Nature study.

"If this type of immunity can be elicited, then in theory a vaccinated person could be protected against any influenza virus including viruses that have yet to emerge from avian or mammalian reservoirs."