A volunteer in a medical study who had an unusual reaction to the flu virus could hold the key to developing a vaccine that protects against all pandemic strains of the infection, scientists say.

Tests on a group of people who had either been infected with or vaccinated against the flu revealed one participant who produced a "super-antibody" that could fight off every strain of influenza A, the virus responsible for mass outbreaks of the illness.

The person produced too little of the super-antibody to make them immune to the flu, but scientists believe they can boost its effect, and use it to make vaccines against the virus.

In preliminary tests, researchers showed that injecting mice and ferrets with the super-antibody protected the animals against doses of influenza that would normally be lethal. The study appears in the journal, Science.

"What we can do now is mass-produce this super-antibody and give it as a therapeutic," said Antonio Lanzavecchia, director of the Institute for Research in Biomedicine in Bellinzone, Switzerland.

"This could be developed to treat any influenza A infection and prevent any possible new pandemic that will come out. We expect it will block not only the strains that circulate in humans but also those that are present in animals," he added.

Steve Gamblin, a co-author of the study and structural biologist at the Medical Research Council's National Institute for Medical Research in north London, said that, if proven to work safely, the antibody might be given directly to hospital staff and other frontline workers to protect them against flu pandemics.

In the longer term, scientists hope to create a vaccine that makes the body launch its own devastating attack on influenza by producing a surge of super-antibodies.

Traditional flu vaccines make the body produce antibodies that target proteins called haemagglutinins on the surface of the influenza virus. But these antibodies focus their attack on the tips of the proteins, which change as the virus mutates. Because the virus evolves so rapidly, flu vaccines have to be reformulated every season.

The super-antibody is different because it latches on to a part of the stem of the haemagglutinin that is shared by all influenza A strains and appears not to mutate. Lanzavecchia calls it the virus's "Achilles heel".

A universal flu vaccine could transform public health by making seasonal jabs obsolete and reducing the impact of fresh outbreaks, including those that spread from animals to humans, like the recent strain of swine flu.

Sir John Skehel, another co-author of the study at the National Institute for Medical Research, said: "Every year millions of people are infected with influenza A viruses and, although the majority of infections are mild, those in vulnerable groups, such as the very old or the very young, may be worse affected and more likely to die or be hospitalised.

"As we saw with the 2009 pandemic, a comparatively mild strain of influenza can place a significant burden on emergency services. Having a universal treatment which can be given in emergency circumstances would be an invaluable asset."