A universal flu vaccine that neutralises all strains of the virus has come a step closer with the discovery of a "super antibody" in a patient's blood.
In tests the antibody disarmed both the main groups of influenza A viruses, which cause a large proportion of seasonal flu outbreaks.
Flu is difficult to control because it presents a shifting "target" to vaccines by mutating into new forms.
As a result, new vaccines have to be developed every year. In the case of a major epidemic or pandemic, the time it takes to produce an effective vaccine may cost lives.
In the past, antibodies have been identified that target a broad range of Group 1 and Group 2 influenza A viruses, but not both.
The new discovery was made by a needle-in-a-haystack technique that involved screening more than 100,000 antibody-producing white blood cells from eight donors.
One donor yielded an antibody known as FI6 that showed neutralising activity against both groups of influenza A virus. Injected into mice and ferrets, the antibody protected against infection by both Group 1 and Group 2 viruses.
The research is reported in the online version of the journal Science.
Dr Steve Gambin, one of the scientists from the Medical Research Council National Institute for Medical Research in London, said: "Historically, it has been impossible to predict precisely what kind of flu could develop into an epidemic and, as such, it has been necessary to develop new vaccines each year to tackle the different viruses. Our discovery may eventually help to develop a universal vaccine."
Co-author Professor Antonio Lanzavecchia, director of the Institute for Research in Biomedicine in Bellinzona, Switzerland, said: "The high prevalence of seasonal influenza and the unpredictability of new pandemics highlight the need for better treatments that target all influenza viruses. As the first and only antibody which targets all known subtypes of the influenza A virus, FI6 represents an important new treatment option and we look forward to taking it through to the next stage of development."