updated 4/7/2008 9:47:27 PM ET 2008-04-08T01:47:27

Chinese health officials have confirmed that a father caught bird flu from his son in December, according to a report released Tuesday.

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Human-to-human transmission of bird flu has happened about a dozen times in the past, in countries including Cambodia, Thailand, Vietnam and Indonesia. In nearly every case, transmission has occurred among blood relatives who have been in close contact, and the virus has not spread into the wider community.

In the case in China, a 52-year-old man and his 24-year-old son in Jiangsu province were diagnosed with H5N1 bird flu within a week of each other in December. At the time, officials from the World Health Organization said they could not rule out the possibility of human-to-human transmission.

After the son died, his father was treated with antivirals and participated in an H5N1 vaccine trial. He survived.

The son's only exposure to bird flu was at a poultry market, while the father apparently had no direct exposure to sick birds. His only known exposure to bird flu was close contact with his ill son.

The H5N1 viruses from the father and son were almost genetically identical.

Experts also tested 91 friends, colleagues, and family members of the father and son — all of whom tested negative for H5N1, proving that the virus is not casually transmitted.

"Limited, non-sustained person to person transmission of H5N1 virus probably occurred in this family cluster," wrote researchers at Beijing's Chinese Centre for Disease Control and Prevention in the medical journal, The Lancet.

"There is no indication from this data that we are any nearer to a pandemic," said Ian Jones, a professor of virology at the University of Reading. Bird flu remains difficult for humans to catch, and experts think most cases are linked to close contact with infected birds.

Health officials monitor every potential case of human to human transmission with particular concern to see if the virus might have mutated into a form that is more easily spread. So far, that has not happened.

"An air of tension still surrounds this disease," wrote Dr. Jeremy Farrar of the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, and colleagues. "Given that the species barrier can be breached, the intriguing question is why the transmissibility of H5N1 among people remains so low?"

Many flu experts worry that H5N1 will spark a pandemic, potentially killing millions worldwide. But despite circulating widely in Asia and beyond since late 2003, the virus only rarely infects humans. As of April 3, WHO reported 378 cases and 238 deaths worldwide.

Last week, the agency confirmed another instance of human-to-human transmission in Pakistan from last December.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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