Chinese health officials have confirmed that a father caught H5N1 bird flu from his son last December, according to a report released Tuesday. This marks at least the fourth instance in which many authorities now believe there was limited inter-family H5N1 transmission.

This latest case also has raised concerns that there could be many undiagnosed human H5N1 infections in China.

WHO and other authorities had been investigating the suspicious H5N1 transmission between members of the family in China since their infefections were first reported. In this case, a 52-year-old man from Nanjing, the capital of the eastern province of Jiangsu, was confirmed to have the virus in his respiratory tract just days after his 24-year-old son died from the virus on Dec. 2. The father recovered following administration of the antiviral Tamiflu at the onset of symptoms and participated in an H5N1 vaccine trial. 

Joanna Brent, a Beijing-based WHO spokesperson had said at the time  that “the possibility of human-to-human transmission cannot be ruled out."

The case was disturbing because WHO and the Jiangsu Animal Husbandry and Veterinary Bureau said there have been no reported H5N1 outbreaks in poultry in the province. A mandatory order to vaccinate poultry has been in effect in the province since 2003, and a recent survey revealed all poultry had been vaccinated and that 92 percent of it had developed antibodies.

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 infected son.

"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.

Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy (CIDRAP) and the National Institutes of Health-supported Center of Excellence for Influenza Research and Surveillance within CIDRAP, had early told HSToday.us that the dramatic spread of “active” virus strains in birds around the planet has become a veritable biological soup for mutations, including a mutation which allows the virus to more easily live in a person's upper respiratory system, like in the case of the father and son in China.

As HSToday.us and HSToday have reported, current testing shows bird flu strains are undergoing rapid and troubling mutations that are pushing the viruses closer to being able to be passed between humans.

Statistically, human-to-human transmission occured in two families in Asia that were studied, said Dr. Ira Longini Jr. of the Seattle, Wash.-based Fred Hutchinson Cancer Research Center.

“The world really may have dodged a bullet … and the next time we might not be so lucky,” Longini said, referring to a study of which he was senior author, published in the September issue of the CDC’s journal, Emerging Infectious Diseases.

Longini and others believe the study indicates a global pandemic was narrowly averted either because of quick action by health authorities or, more disturbingly, statistical good luck. Either way, scientists say the statistically relevant data confirm their private fears that some strains of H5N1 have been able to mutate into human transmissible form.

But in each of the cases of human to human transmission, there was no transmission outside the family, leading some researchers to suspect that some genetic predisposition to the virus may be in play.

Jeremy Farrar of the University of Oxford's Clinical Research Unit in Ho Chi Minh City, Vietnam, was quoted by New Scientist saying, "there may be a very rare genetic predisposition to the virus" that involves human genes for receptors that can dock with H5N1 in the respiratory tract, or involves different variants of the virus.

"Or it may be a particular host and a particular virus 'like each other' to the extent that the same virus may not be able to infect another, unrelated individual," Farrar said.