Bird flu vaccine may be available to more people in the event of a pandemic after researchers engineered a way to make stocks go further, it has emerged.
Combining the vaccine with a special solution makes it six times more effective, meaning much less vaccine is needed to protect individuals.
The research team from Belgium told The Lancet that the vaccine is also effective against different strains.
Experts said the results were unexpected but "very good news".
Influenza vaccines are a central part of worldwide plans to control the spread of bird flu in the event of a pandemic, but vast numbers of doses will be needed.
And production capacity of influenza is limited - it takes time to grow vaccine strains in the laboratory.
Dose reduction
The latest study found that placing the vaccine in a special oil-in-water emulsion, or "adjuvant", greatly improved the immune response in humans.
In fact, the dose needed for a good response was almost four times lower than that used normally in influenza vaccines, meaning limited supplies would stretch to more people.
The researchers also found the vaccine produced immune responses against H5N1 subtypes from Vietnam and Indonesia.
"We were all very pleased to see the results, and very surprised"
Potentially, the vaccine, made by GSK, could be produced in advance before it was known what strain was causing a pandemic, they said.
The study leader, Professor Geert Leroux-Roels of Ghent University, said reducing the dose needed was just one of the strategies being looked at to make sure as many people could be vaccinated as possible.
"You can reduce the dose from 15 micrograms to 3.8 micrograms which would allow you to make four times as many vaccines."
He added that there were several ways the adjuvant may help - the oil-and-water mix alters how the influenza protein is released into the body and is also thought to have a direct effect on attracting immune system cells to the injection site.
Professor John Oxford, virologist at Queen Mary's School of Medicine and Dentistry in London, said it was very good news.
"We were all very pleased to see the results, and very surprised that the adjuvant broadened the immune response and range of avian influenza viruses.
"The results exceeded expectations, but it's not crystal clear how the adjuvant works."
Two vaccine companies - Baxter and GSK - have just received contracts worth £155.4 million from the government to supply a pandemic influenza vaccine as soon as the pandemic strain is identified and made available by the World Health Organization.
Professor Oxford added that having the guaranteed order in place meant the UK would be at the top of the list in the "scramble" to get hold of vaccine.
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