CDC Says Bird Flu Virus Is Stable

WASHINGTON (AP) -- Genetic studies of a bird flu that has killed six people in Hong Kong show that it has not mutated into a more dangerous virus and is unlikely to cause a worldwide epidemic.
Federal experts, however, say there remains a need for caution. Just to be on the safe side, scientists at the Centers for Disease Control and Prevention are working on a new vaccine.
A report on the genetic study of the bird flu virus appears today in the journal Science.
``It is good news for now,'' Dr. Kanta Subbarao of the CDC flu laboratory in Atlanta said. She said experts, though, worry that the Hong Kong bird flu still could make genetic changes that would turn it into a more virulent human killer.
``We remain concerned that a reassortment (genetic change) could occur at any time,'' said Subbarao, the first author of the study.
A 25-year-old woman died Wednesday, the Hong Kong government announced Thursday. She was the sixth person in Hong Kong killed by the bird flu since the outbreak began in May. At least 12 other people have been infected.
Subbarao said she and other CDC experts have not determined how the avian flu virus strain, called H5N1, managed to infect humans. Usually a bird flu virus will not directly spread to humans, and the fact that H5N1 made some people sick has alarmed international flu experts, she said.
Flu virus genes are highly changeable, and slightly altered flu strains appear almost annually. Usually, the new strain resembles previous strains and most humans have some immunity against such viruses.
Occasionally, a unique strain develops that is particularly deadly. Many such flu strains originate in birds, then spread to swine, which are capable of hosting both the bird flu and human flu. When the two viruses genetically mix in the pig, the result can be a totally new flu strain.
There were fears, said Subbarao, that this genetic mixing could have happened to H5N1 inside humans who got the bird flu in Hong Kong. She said a bird flu that acquired internal human flu genes could be capable of spreading rapidly from person to person.
``The human population would have no immunity, no protection against such a virus,'' said Subbarao. ``The virus could spread and cause a pandemic (a worldwide epidemic). ... It could have a very high mortality rate.''
A unique flu strain in 1918 killed 20 million people worldwide. Smaller outbreaks occurred in 1957 and 1968.
Subbarao said studies at the CDC found that the H5N1 that infected 18 people in Hong Kong has not changed genetically. All of its genes are those of bird flu.
This means, she said, that H5N1 is not now highly virulent among humans, boosting confidence that the immediate human threat is over.
To contain the virus, health officials slaughtered more than a million chickens in Hong Kong and blocked importation of chickens from nearby provinces in China.
Clearing Hong Kong of live chickens, said Subbarao, may have ended the outbreak, at least for now.
``The slaughter was absolutely essential,'' virus expert Robert Webster of St. Jude Children's Research Hospital said in Science. ``The big question is whether the stable door was shut in time.''
Subbarao said no new human case of H5N1 has been recorded since Dec. 28. No cases have been reported outside Hong Kong.

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