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Study: Bird-flu vaccine safe, but only partly effective

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(CNN) -- An experimental bird-flu vaccine for humans has been deemed safe, but it provides only partial protection from the H5N1 strain of the virus, according to a study published this week.

The vaccine, which the United States already is stockpiling in the event of a pandemic, was given to 451 healthy adults between the ages 18 and 64, making it the largest such study to date.

A pandemic, or widespread outbreak, could occur if the virus mutated to the point that it was easily spread from person to person. So far, it has not.

Of the participants given two shots of the highest dose of the vaccine 28 days apart, only 54 percent of them had adequate protection from the virus when researchers tested their blood for antibody response. Not all participants were given the same dosage.

Researchers were trying to determine if the vaccine is safe and what dosage would provide sufficient protection from the virus should it become pandemic. The ongoing clinical trials are being funded by the National Institutes of Health.

Smaller studies have shown the vaccine is safe and well tolerated by children and the elderly.

"Mild pain at the injection site was the most common adverse event for all doses of vaccine," the study found.

But the study, published in the March 30 edition of the New England Journal of Medicine, shows that while the shot is safe, only about half of those receiving the highest dose of the vaccine -- 90 micrograms -- produced enough antibodies to stave off H5N1 bird flu, if they were exposed.

The seasonal flu shot, by comparison, provides adequate protection in 70 to 90 percent of those receiving one 15-microgram dose of the vaccine.

"We recognize these are lower response rates than you would see in a normal flu vaccine," Dr. John Treanor told reporters in a telephone briefing Tuesday. Treanor is director of the University of Rochester's Vaccine and Treatment Evaluation Unit and lead author of the study.

Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases, which funded the research, concurred with Treanor.

"It's a bit of a muted good news, but we have a long way to go," he said.

Because the required dose is so high and two shots are needed, the nation's H5N1 vaccine stockpile of about 8 million doses is enough for about 4 million people.

"We have to hold on to this vaccine and work on a new vaccine," said Dr. William Schaffner, of Vanderbilt University's School of Medicine, who was on the data safety monitoring board for this vaccine. He added that the vaccine, though imperfect, provides a foundation for the development of other vaccines as the virus mutates.

Because the supply is limited, those manufacturing the vaccine and caring for the sick will likely get the first inoculations in the event of a pandemic, Fauci said. Manufacturers can't make more of the vaccine, which takes six months, until the companies finish producing seasonal flu vaccine for the 2006-2007 season, he said.

"It may not protect against infection but could have a stop-gap effect in spread of disease," Fauci said, emphasizing that the current vaccine is meant for a pre-pandemic situation.

If the virus were to change in a way that made it easily transmissible from person to person, then a new vaccine based on that strain would need to be be developed.

At this point, the deadly H5N1 virus that's infected millions of birds worldwide is not easily transmitted from person to person. Worldwide, 186 people are known to have been infected with H5N1, of which 105 have died, according to the World Health Organization.

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