Human trials indicate an H5N1 bird flu vaccine developed using a virus isolated in Vietnam can neutralize antibodies from H5N1 viruses found in other countries, the vaccine's manufacturer said Wednesday.

The preliminary trial results raised hopes that vaccines based on older H5N1 bird flu strains might prove effective against future variants of the virus in the event of a pandemic.

In Sanofi Pasteur's trial, 300 volunteers were vaccinated with a strain of the virus isolated in Vietnam in 2004. Antibodies were then examined from their blood, and tests were done using H5N1 viruses from Turkey and Indonesia. The results indicated that the volunteers' antibodies were able to neutralize the other H5N1 viruses, proving that some measure of cross-protection is possible.

"This is a milestone for vaccine development," said Dr. Klaus Stohr, the World Health Organization's top official on pandemic influenza vaccines.

Stohr said that while tests in mice and ferrets had suggested that cross-protection might be possible, this is the first evidence available from human trials.

The H5N1 strain of bird flu has been circulating since late 2003 in Asia. Last year, it spread to parts of Europe and Africa. Flu experts believe it is the leading candidate to ignite the next flu pandemic. To date, WHO has recorded 151 deaths worldwide from H5N1.

More than a dozen companies are working on possible pandemic vaccines, based on the Vietnamese 2004 H5N1 strain.

However, tests need to be conducted on these candidate vaccines to determine if they would also protect against newer versions of the virus.

"We don't know enough right now to make any blanket statements about cross protection," said Dr. John Treanor, a flu vaccine expert at the University of Rochester. Sanofi Pasteur's initial results, however, could be reassuring for governments investing in vaccine stockpiles, Treanor said.

"This tells us that stockpiling does make sense," Stohr said. "It gives us another avenue of pandemic preparedness," Stohr said, suggesting that people could be inoculated with a pre-pandemic vaccine, before being given a booster shot once the pandemic strain emerges.

Still, an effective H5N1 vaccine that protects against other H5N1 strains does not guarantee protection against a pandemic. Such a vaccine could offer no protection against other subtypes of influenza.

"We don't know if the next pandemic will be started by H5," said Dr. Angus Nicoll, director of influenza coordination at the European Centre for Disease Prevention and Control.

"It's still a very difficult risk judgment," says Nicoll. "While it's good to have a vaccine that offers cross-protection, it is not the ultimate answer."