WASHINGTON (Reuters) - U.S. scientists said on Thursday they have developed a vaccine against the Hong Kong "bird flu" -- an unusually deadly form of influenza that has badly frightened health officials.

The flu, known by its genetic designation H5N1, has killed eight people and infected just over 20 in two outbreaks, but its deadliness prompted Hong Kong health officials to slaughter millions of chickens in 1997.

The team at St. Jude Children's Research Hospital in Memphis, Tennessee, said they used a type of genetic tinkering called reverse genetics to weaken the virus and make a vaccine.

They want to rush it into human trials, in case the new flu makes another comeback, and say the current outbreak of the flu-like disease known as severe acute respiratory syndrome (SARS) in Hong Kong and elsewhere illustrates how easily new infectious diseases can appear.

So far, H5N1 influenza does not appear to spread from person to person, but rather appears to jump directly from birds to people. But it has already mutated to infect several new bird species and could continue to change, said Dr. Robert Webster, a flu expert at St. Jude.

"It's important to move right along with these trials in case the virus begins spreading from person to person," Webster said in a statement.

H5N1 flu first showed up in Hong Kong in 1997 and was quickly traced to the territory's crowded poultry markets. It killed six people and infected 18 before it was stopped.

"It's likely there were two things that prevented the 1997 poultry influenza outbreak in Hong Kong from becoming more deadly -- its inability to spread from human to human and the slaughter of more than 1.5 million chickens and other birds in the open-air markets of Hong Kong, which eliminated the source of the virus," Webster said.

"In fact, the sudden appearance of SARS in the same region of the world is just another warning that the large populations of people and poultry in this region are a potential source of viruses," he added.

H5N1 made its next appearance a few months ago, causing authorities to slaughter several thousand more chickens. Then it killed ducks and flamingos in some of Hong Kong's parks.

Last February, H5N1 flu killed two members of a Hong Kong family who visited Fujian Province in southern China.

St. Jude's Richard Webby, who helped develop the vaccine, found the string of amino acids that make the most toxic part of the virus and figured out how to remove them. He replaced the genetic code with a similar code from the less-dangerous H1N1 "master" strain of virus commonly used to make vaccines.

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