Some chickens infected with the virus did not show any symptoms of illness. 'Among poultry ... the virus is causing some milder asymptomatic infections. That is of course worrisome... It's more difficult to control the infection in poultry.'

Starting Aug. 1, commercial poultry operations and smaller household farms in northern Nam Dinh province and southern Tien Giang province in the Mekong Delta will be vaccinated, said Bui Quang Anh, head of Vietnam's animal health department.

Vaccinations will be slowly expanded to another 40 high-risk provinces over the next two years, he said. An initial 20 million doses of vaccines will be imported from the Netherlands and China.

'Good Preparation Will Be Key'

Bird flu began ravaging poultry farms across Vietnam in late 2003, killing or forcing the slaughter of more than 45 million birds. The virus began jumping to humans at about the same time, and has killed 38 people in Vietnam, 12 in Thailand and four from Cambodia.

"Good preparation will be key to the success of this effort," Anh said during a press conference Wednesday. "We cannot afford to allow something bad to happen. If the vaccinations are not closely monitored, the virus could change."

Most human cases have been linked to contact with sick birds. But health experts worry the virus will mutate over time and spread easily from person-to-person, potentially causing a pandemic that could kill millions worldwide.

A team of virologists and epidemiologists from Hong Kong, Japan, Britain and the United States in Vietnam last week discovered no changes in the virus' form, according to the World Health Organization, which coordinated their visit.

"That's very good news," said Hans Troedsson, head of the WHO in Hanoi, adding that experts "also concluded that they could not verify that the virus was spreading among people...They did not detect new cases."

Rate of Mortality

There were some suspicions that the H5N1 strain of bird flu virus had changed because of recent clusters of human cases where patients showed no symptoms of illness. The rate of mortality in bird flu patients has also been much lower than in previous outbreaks.

Earlier this week, the UN Food and Agriculture Organization said test results showed that some chickens infected with the virus did not show any symptoms of illness.

"Among poultry, this is correct that the virus is causing some milder asymptomatic infections. That is of course worrisome... It's more difficult to control the infection in poultry," Troedsson said.

The two-year poultry vaccination program will cost a total of 560 billion dong (US$35 million, €29 million), said To Long Thanh, also with Vietnam's Department of Animal Health.

The government will subsidize 460 billion dong (US$29 million, €24 million) and the remainder will come from fees charged to large commercial operators. Household farmers will get their chickens vaccinated for free, he said.

Previously ducks and other migratory birds had been the biggest concern in the spread of bird flu since they were shown to carry the virus without becoming ill.

Vietnam has ordered that all ducks and other waterfowl be culled if they test positive for the virus. However, the government estimates that only 10 percent of infected waterfowl have been destroyed because farmers have balked at slaughtering their flocks due to low compensation.

The government announced an increase in compensation this week, from 5000 dong (30 U.S. cents, 24 euro cents) to 15,000 dong (US$1, 80 euro cents), in the hopes that an estimated 10 million waterfowl will be culled in the coming months.