As China prepares to inoculate billions of birds against avian influenza, international animal health specialists are warning that improper vaccination of chickens and ducks in East Asia may be helping to spread the virus, increasing the risk of a human outbreak.

Three of the countries hardest hit by the lethal disease -- China, Vietnam and Indonesia -- are focusing on immunizing livestock, but health officials express concern that the use of fake or substandard vaccines could leave birds unprotected or, even worse, infect healthy ones.

Earlier this month, Chinese agriculture officials reported that unlicensed vaccines had been sold to farmers in the northeastern province of Liaoning, causing what chief government veterinarian Jia Youling called "incalculable" harm. Officials have reported four outbreaks in Liaoning in recent weeks. More than 10 million birds have been destroyed to combat the disease, and health officials are now investigating whether a local poultry worker contracted it.

Farmers in Liaoning had used a vaccine approved by the government solely for testing, according to Hong Kong's South China Morning Post. The Shanghai-registered manufacturer issued a statement saying its salesmen had smuggled the vaccine to the province and sold it in a market.

After the Agriculture Ministry's disclosure, U.N. officials in Beijing asked the government for details, including specifics about the vaccines used. Roy Wadia, spokesman for the World Health Organization in Beijing, said the government has not responded.

The concern about substandard medicines assumed greater urgency last week when Chinese officials announced they were extending their inoculation campaign to cover every chicken and duck in the country, which by some estimates accounts for a quarter of the world's poultry.

"Especially in the current situation, when you're going to vaccinate the whole country, it's very, very important you use high-quality vaccines," said Christianne Bruschke of the World Organization for Animal Health in Paris. With so many birds to be immunized in China, Bruschke said there was a danger that the capacity to produce vaccines would be stretched thin, increasing the prospect of substandard production.

While fake vaccines leave poultry unprotected, a low-quality vaccine with substandard potency could be even more hazardous because it can mask the symptoms of bird flu without blocking the virus. Chickens and ducks become silent spreaders, able to evade quarantine and other measures meant to stem the disease.

Another problem is the improper manufacturing of vaccines, in which live viruses used to make them are not completely inactivated. The inoculations infect the birds, which then become carriers. International health experts said they have high regard for China's official research institute in Harbin, which produces several types of vaccines, but they suspect that many other vaccines are marketed by Chinese manufacturers of questionable reliability.

At least 130 people in East Asia have been diagnosed with bird flu, and 67 have died since 2003. In most cases, they contracted the disease through direct contact with infected poultry. Influenza specialists fear the virus could develop into a form easily transmitted among humans, which they say would spark a pandemic.

In Vietnam, vaccine safety is also a concern; the government is seeking to inoculate as many as 150 million chickens and ducks by next month with doses imported from China.

"We have a concern about the quality of the Chinese vaccines being used," said Patrice Gautier, representative of Veterinarians Without Borders in Hanoi. "Nobody has enough information about the content of the vaccine. There is no test being done on the vaccine on a regular basis."

He added that he also has misgivings about how the immunization campaign is being conducted. In a country where millions of chickens and ducks roam free in villagers' back yards, Gautier said many birds would not receive the two injections required for full protection. This could leave poultry infected and able to spread the disease while showing no symptoms.

In Indonesia, officials said they have embarked on a nationwide poultry immunization program because they do not have enough money to compensate farmers for culling flocks in infected areas. Two-thirds of Indonesia's provinces have registered outbreaks in birds.

Last month, Indonesian agriculture officials reported that they had discovered backyard chickens that appeared to be healthy but were infected. Animal health experts in several Asian countries suggested this was the result of faulty vaccines and warned of the heightened risk this posed to people.

"It caused a great degree of concern," said Jeffrey Gilbert, a WHO expert on animal-based diseases.