Specialists worry the virus could spread and mutate in China because of its huge poultry flocks and their contact with humans. It also has migration routes for geese and other wild birds that might carry the disease.
''This is a psychologically telling moment for a country that has never had bird flu cases in the past in humans," said Roy Wadia, a WHO spokesman in Beijing. ''This will drive home to citizens across the country that this can happen in our own backyards. It's a very real threat."
Officials had warned that a human infection in China was inevitable after the country suffered 11 outbreaks in poultry over the past month, which prompted authorities to destroy millions of birds.
Elsewhere in Asia, the H5N1 strain has infected at least 126 people and killed at least 64 of them since 2003, two-thirds of them in Vietnam.
Nevertheless, WHO spokeswoman Maria Cheng in Geneva said the Chinese cases do not increase the risk of a flu pandemic because there has been no observed genetic change in the virus and no apparent spread between people.
She said it would not be surprising if more human bird flu cases are confirmed in China. ''There are a lot of chickens infected and there's a lot of contact between humans and chickens in China," she said.
The Chinese government announced plans Tuesday to vaccinate all the country's 14 billion domestic fowl.
It was not clear how long that would take. According to Chinese health officials, vaccinating chickens can require repeated injections and booster shots. State television showed workers at industrial-scale poultry farms jabbing chickens with injector guns.
Health specialists in Geneva said shots were the most reliable way to deliver vaccine, although it can also be administered by mixing it in the animals' feed.
Officials in Liaoning in China's northeast, the site of four outbreaks, said they have finished a vaccination program begun this month for the province's 320 million birds.
