The World Health Organization on Friday reported the first lab-confirmed case of avian flu spreading from one person to another - a 10-year-old boy in Indonesia who infected his father.

But health experts said there's no immediate cause for concern. Although this flu strain's unique genetics made it relatively easy to trace from person to person, it died with its victims and was no more likely to spread between humans than other strains.

“If anything, it appears we have ducked another bullet,” said Dr. William Schaffner, an avian flu expert and chairman of preventive medicine at Vanderbilt University Medical Center.

Since the first human cases of avian flu were reported in Southeast Asia in 1997, experts have warned that the constantly mutating virus someday could spawn a worldwide epidemic, known as a pandemic.

The virus has killed millions of birds in at least 30 countries, but outbreaks among humans have been relatively rare.

There have been 228 reported human cases worldwide and 130 deaths, almost all of them in Asia, according to the World Health Organization.

Health experts say the fact that avian flu kills more than half its victims makes the prospect of a pandemic particularly frightening.

In the Indonesian case, seven members of a family were infected on a remote northern island, according to Dick Thompson, a spokesman for the World Health Organization. Six of the victims died.

The first five relatives were infected with identical strains of H5N1, as the bird flu strain is known. But the virus mutated in the sixth victim, a 10-year-old boy. He passed the mutated virus to his father, Thompson said. That mutation allowed an Indonesian lab to match the strains that infected father and son, he said.

The father died about four weeks ago, he said. But soon after he was infected, roughly 50 people that he had come into contact with were identified and closely observed for three weeks - twice the normal incubation period for avian flu. None developed avian flu symptoms, Thompson said.

The mutation made the avian flu strain easier to identify, but did not make it any more infectious, Thompson said.

Health experts say that human-to-human transmissions of avian flu - within families - have occurred before. The first reported case was published last year in the New England Journal of Medicine, when researchers documented daughter in Thailand who infected her mother.

Family clusters of avian flu victims also have been found in Azerbaijan, Turkey and Thailand, said Dr. Robert Edelman, associate director of the Center for Vaccine development at the University of Maryland School of Medicine, one of the current testing sites for an avian flu vaccine.

Edelman is part of a national effort to come up with an experimental avian flu vaccine that could be distributed in the event of an outbreak.

Initial results of a vaccine tested last year in a major clinical trial at research sites around the country were mixed. Researchers announced in March that the vaccine triggered protective immune responses in about half of those given a high dosage.

The vaccine appeared to be safe, but required such large doses that it would be difficult to make sufficient quantities for a major outbreak, the researchers say.

Edelman and other researchers hope an updated vaccine using aluminum hydroxide to fortify its effects will yield better results.

The second vaccine is being tested nationwide on about 600 adults and another 600 people age 65 and older in a second clinical trials.

Distributed by the Los Angeles Times-Washington Post News Service

AP-NY-06-23-06 1930EDT