Hungary's health minister says a bird flu vaccine appears to be effective in early tests.
The vaccine works against H5N1
The trial jab appears to protect humans and animals against the lethal H5N1 virus, preliminary results show.
Testing of H5N1 vaccines are also is under way in other countries, including Britain and the US.
However, the real fear is that H5N1 will mutate and trigger a human pandemic and such trial vaccines may not work in these circumstances.
Experts estimate that it would take four-to-six months from the time a pandemic flu strain emerges to develop and manufacture a bespoke vaccine.
At present, H5N1 flu strain poses only a limited threat to humans as it cannot spread easily between people.
Experts say it is only a matter of time before the strain acquires this ability, causing a flu pandemic which could kill as many as 50,000 people in the UK.
Hungary's health minister Jenö Rácz was among several dozen Hungarians who underwent tests of the trial H5N1 vaccine.
He said: "The results are preliminary but I can say with 99.9% certainty that the vaccine works."
UK ministers have arranged for 2-3 million doses of a H5N1 vaccine to be available, which could offer some protection against the virus.
This could be given to people at particularly high risk of infection, such as health care workers.
Researchers would also be able to carry out further clinical studies on the H5N1 vaccine to learn more about how it works against the virus and how effective it could be.
The Department of Health is also stockpiling 14.6 million doses of the antiviral drug Tamiflu which works by reducing the symptoms and the risk of a carrier passing on the virus.
Once the exact pandemic strain was known, enough vaccine would be purchased to cover every person in the UK, Chief Medical Officer Sir Liam Donaldson has promised.
He estimates 120 million doses would be needed - two for everyone.
Professor Ian Jones, a virology expert at the University of Reading, said whilst the news of the Hungarian vaccine was encouraging, "it does not mean it would be useful in the event of a true pandemic."