Scientists from Hong Kong and the United States say they have developed an experimental bird flu vaccine by modifying a vaccine once used to treat smallpox.

They told a news conference in Hong Kong Sunday that initial tests on mice showed the new vaccine to be highly effective.

An article published in the current "Journal of Immunology" explains how the scientists from Hong Kong University and the U.S. National Institutes of Health inserted key components of the H5N1 bird flu virus into the smallpox vaccine.

The scientists say the new bird flu vaccine could be produced cheaply and would have a long shelf-life, making it more accessible to poorer nations.

But they say it would not be ready for sale for a few more years, because it has not yet been tested on humans.

The World Health Organization says 408 people have contracted bird flu worldwide since 2003. More than half of the victims died.

Scientists fear that if the H5N1 bird flu virus mutates into a form more easily passed among humans, a worldwide epidemic could kill millions of people.

Some information for this report was provided by Reuters.