The total number of verified human cases since the 2003 outbreak began is 391, of whom 247 died. After peaking in 2006 at 115 human cases with 79 deaths, human infections dropped to 40 in 2008, with 30 deaths, according to a World Health Organization update in mid-December.
Most of the human cases were traced to direct contact with poultry, especially in Southeast Asia where many people have backyard flocks and few wear gloves or masks while handling them. The few suspected human-to-human transmissions occurred in those who were closely involved in caring for an infected relative.
But as long as the virus continues to circulate, the threat that it could mutate to pass more easily among humans remains, according to the U.N. report.
The Hong Kong poultry outbreak last month is significant because the government thought it had stamped out H5N1 in the Chinese territory after an outbreak in 2003. Since then, Hong Kong has vaccinated poultry against the virus and strictly regulated farm sanitation.
The government ordered the slaughter of 80,000 fowl at two large farms after the latest outbreak killed 60 chickens at one of the farms. Investigators are looking for the source of the infection and testing the effectiveness of the vaccine used since 2003 to inoculate chickens, geese and ducks against H5N1.
Hong Kong uses a vaccine that protects poultry against several flu subtypes. But some scientists believe that the H5N1 virus may have mutated to break through the vaccine. Flu viruses change constantly, which is why human vaccines for seasonal flu are modified every year, said Scott P. Layne, a professor of epidemiology and environmental health sciences at UCLA.
Mainland China is using a newer poultry vaccine developed specifically for H5N1. But vaccination programs there and in Vietnam have not eliminated outbreaks.
The vaccine itself could be the problem, said Robert Webster, a virologist and avian flu expert at St. Jude Children's Research Hospital in Memphis, Tenn.
Vaccines should be used only in areas where the virus is out of control, and then only temporarily, he said. That is because routinely administering the vaccine encourages the evolution to resistant strains.
Some countries have managed to stop the virus by culling infected poultry flocks. Japan, South Korea and Malaysia are considered to be free of H5N1, according to the World Health Organization.
But the virus appears to be entrenched in Indonesia, parts of China, Vietnam, Egypt and other countries where backyard flocks are more difficult to regulate than commercial chicken farms, according to the United Nations' Food and Agriculture Organization.
Though bird flu viruses are common, highly pathological ones such as the 1918 virus and H5N1 -- which has been lethal to 100% of chickens infected and 63% of humans known to be infected -- are rare.
Scientists have little experience with which to gauge how H5N1 will evolve.
But, Webster said, "We still have to treat this as a potentially very, very dangerous virus."
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mary.engel@latimes.com
