The first antibody which can fight all types of the influenza A virus has been discovered, researchers claim.
Experiments on flu-infected mice, published in Science Express, showed the antibody could be used as an "emergency treatment".
It is hoped the development will lead to a "universal vaccine" - currently a new jab has to be made for each winter as viruses change.
Virologists described the finding as a "good step forward".
Many research groups around the world are trying to develop a universal vaccine. They need to attack something common to all influenza which does not change or mutate.
It has already been suggested that some people who had swine flu may develop 'super immunity' to other infections.
Scientists from the Medical Research Council's National Institute for Medical Research at Mill Hill and colleagues in Switzerland looked at more than 100,000 samples of immune cells from patients who had flu or a flu vaccine.
They isolated an antibody - called FI6 - which targeted a protein found on the surface of all influenza A viruses called haemagglutinin.
Sir John Skehel, MRC scientist at Mill Hill, said: "We've tried every subtype of influenza A and it interacts with them all.
"We eventually hope it can be used as a therapy by injecting the antibody to stop the infection."
Professor Antonio Lanzavecchia, director of the Institute for Research in Biomedicine, Switzerland, said: "As the first and only antibody which targets all known subtypes of the influenza A virus, FI6 represents an important new treatment option."
When mice were given FI6, the antibody was "fully protective" against a later lethal doses of H1N1 virus.
Mice injected with the antibody up to two days after being given a lethal dose of the virus recovered and survived.
This is only the antibody, however, not the vaccine.
A vaccine would need to trigger the human body's immune system to produce the antibody itself.
Sir John said the structure of the antibody and how it interacted with haemagglutinin had been worked out, which would help in the search for a vaccine, but that was "definitely years away".
Professor John Oxford, a virologist at Queen Mary, University of London, said: "It's pretty good if you've got one against the whole shebang, that's a good step forward."