The trials will help doctors prepare for a pandemic
A vaccine being tested right now could help save lives in a future bird flu pandemic, UK scientists claim.
A jab against one strain of avian flu, given years earlier, may "prime" the immune system to fight a wide range of bird flu strains.
When the pandemic arrives, "pre-vaccinated" people could then be given a booster shot, and be protected far quicker, said researchers.
The research was published in the New England Journal of Medicine.
The speed that pandemic flu - labelled the "gravest threat" to the UK by a recent government document - could sweep the world, is one of the great challenges facing scientists and governments.
Some suggestions say it might only be a matter of weeks before an emerging virus reached the UK.
By the time a vaccine exactly matching the pandemic strain is developed and administered, it may already have claimed many thousands of lives.
Unlike seasonal flu, to which most of us have been exposed at some point, most humans will have far less immunity to pandemic flu, and this means that multiple vaccinations over a period of at least a month will be needed, in addition to a delay of weeks while antibodies tailored to fight the strain are built up.
The Leicester researchers say that their solution could mean that a single vaccination of the pandemic strain vaccine would be needed, and it could be effective within a week.
The research centred on people given a vaccine against the H5N3 strain of bird flu between 1999 and 2001.
The vaccine contained another ingredient called MF59 designed to boost its effectiveness.
Years later, they were vaccinated against the H5N1 strain of avian flu, and their immune system response compared against a group of people who had not received the earlier vaccination.
After just seven days, 80% of the "primed" group had signs that their body was protected against H5N1, compared with 20% of the "unprimed" group.
The earlier vaccine had not only offered protection against that strain, but laid the foundations for protection against other avian flu strains, said the researchers.
No time to lose
Dr Iain Stephenson, one of them, said: "If a bird flu pandemic erupted tomorrow it isn't clear we would have six weeks to vaccinate people before it arrived in this country, even if the vaccine was stockpiled.
"We have been able to prove in this study that you can vaccinate people six, seven, or eight years ago and still get a very rapid response with a booster shot within a week."
Dr John Wood, from the National Institute for Biological Standards and Control, a government funded body which helps in the production and testing of vaccines for emerging flu strains, backed the study.
"The fact that they seem to have this protection after eight years is really interesting," he said.
"If governments are thinking about stockpiling vaccine, you could actually be stockpiling it in people's arms."
He said that while there was no certainty that any flu pandemic would be based on avian flu, the Leicester research should be followed up, with the possibility that people might eventually be primed against a "cocktail" of different flu types.