WASHINGTON: Human tests of an experimental vaccine to treat bird flu have shown it is effective at fighting the virus strain that experts worry could spur a worldwide pandemic.
Health officials have been racing to develop a vaccine because they fear that if the avian influenza strain mutates and combines with a human flu virus to create a new virus it could spread rapidly throughout the world.

US National Institute of Allergy and Infectious Diseases director Anthony Fauci said the findings were a step forward but did not overcome the major hurdle of producing enough vaccine to meet demand in the event of a flu pandemic.

The vaccine is grown in chicken eggs and production can take months.

Dr Fauci said early analysis of tests in healthy adults under 65 showed the vaccine, which is made by French company Sanofi-Aventis, produced a strong immune response.

"It's an important landmark in the broader plan of how you prepare a nation for pandemic flu," Dr Fauci said.

"One of the sobering issues is we still haven't solved the problem that we have had for years and years, which is vaccine-production capability. Are we going to be able to make enough of this stuff?"

The H5N1 strain has killed more than 50 people in Asia since 2003.

Public health experts say the virus is mutating and could develop the ability to spread easily from person to person and kill millions in a flu pandemic.

Officials are working with vaccine makers to try to find ways to produce the vaccine more quickly.

US government scientists tested the Sanofi-Aventis vaccine in about 450 healthy adults under 65 and had analysed data on about 113 of them, Dr Fauci said.

The results showed the doses needed to produce an immune response that would be expected to protect against infection were higher than what is usually given in an annual flu shot, he said.

The test results were first reported by The New York Times.

An earlier human vaccine against the A(H5N1) avian influenza virus was prepared after it first appeared, in Hong Kong in 1997. That vaccine was never fully developed or used, and the strain has mutated since then.

Tests in people 65 and older should begin within a month or so, Dr Fauci said. If those go well, additional tests are expected in children.

The US Government has ordered 2 million doses of the vaccine from Sanofi-Aventis for a national stockpile and is negotiating with the company to order more, Dr Fauci said.

Only a small number of human cases of A(H5N1) influenza have been found.

Although a few cases might have been transmitted from person to person in Asia, the A(H5N1) strain had not garnered enough strength to spread widely among humans anywhere, the Times reported.

According to the World Health Organisation, the avian strain has killed 57 of the 112 people it has been known to infect in four countries -- four cases in Cambodia, one in Indonesia, 17 in Thailand and 90 in Vietnam.

Tens of millions of birds have died from infection with the virus and culling to prevent its spread.