Beijing ? China reported its first human cases of bird flu on the mainland Wednesday, including at least one fatality, as health workers armed with vaccine and disinfectant raced to inoculate billions of chickens and other poultry in a massive campaign to contain the virus.

The World Health Organization confirmed the virulent strain experts fear could cause a worldwide flu pandemic has now infected humans in the world’s most populous nation.

China’s Health Ministry reported confirmed cases of infection with the deadly H5N1 strain in a poultry worker, who died, and a 9-year-old boy, who fell ill in central Hunan province but recovered, the official Xinhua News Agency said. It said the boy’s 12-year-old sister, who died, was recorded as a suspected case.

Experts worry the virus could spread and mutate in China due to 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,” he said. “It’s a very real threat.”

Officials had warned 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 wasn’t 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 experts 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, scene of four outbreaks, said they have finished a vaccination program begun this month for the province’s 320 million birds.

Such vaccination programs are “the right thing to do,” said David Nabarro, the U.N. coordinator for bird and human flu. The virus is so entrenched in China’s birds that simply slaughtering them will not work, he said. The best plan is to vaccinate and then slaughter when there are outbreaks, he said at a conference on bird flu in New York.

China’s prompt response to bird flu and the scale of its anti-disease effort have been in striking contrast to its handling of severe acute respiratory syndrome in 2003, when it was criticized for its secrecy and failure to respond to foreign pleas for information and cooperation.

Since the SARS outbreak, the government has set up disease testing laboratories and a health warning network. It has promised to be more open about epidemics and to cooperate with other nations.

The Chinese territory of Hong Kong recorded the first known cases of human infection with H5N1 bird flu in 1997, when it infected 18 people and killed six, according to WHO. The entire poultry population of about 1.5 million birds was slaughtered.

Chinese officials initially said the 12-year-old girl who died in Hunan tested negative for the virus, as did her brother and a schoolteacher who fell ill at the same time. But the government later asked WHO to help re-examine the case.

Wadia said Chinese investigators were confident the girl died of bird flu, but she couldn’t be considered a confirmed case under WHO guidelines because her body was cremated and there weren’t adequate samples for testing.

The 24-year-old poultry worker died in the eastern province of Anhui, where there was an outbreak last month. But Wadia said the victim didn’t live near that site and instead had contact with birds that died in her own village.

“She died in a hospital,” he said. “She was therefore tested adequately.”

In Liaoning, officials took reporters Wednesday to the village of Qitaizi in an effort to reassure the public by showing off anti-disease work.

Officials destroyed 160,000 chickens in the village after 40 were found dead of bird flu on Nov. 4.

“Obviously there’s no way we can kill all the migratory birds,” said Shao Chuanming of the provincial animal health bureau. “But as long as we can sever the links of transmission between migratory birds, poultry and people, then the controls are effective.”

Also Wednesday, Vietnamese authorities reported bird flu outbreaks in three more provinces, bringing to 12 the number of cities and provinces affected recently. Vietnam is in the middle of a campaign to destroy all poultry in most of Ho Chi Minh City and Hanoi, its two biggest cities.

Ministers at the Asia-Pacific Economic Cooperation summit in Busan, South Korea, urged more information-sharing and response systems to combat bird flu. “New global pandemics, like avian influenza, require new, concerted action,” said Secretary of State Condoleezza Rice.