The Vietnamese government said it had halted the nationwide vaccination of poultry against bird flu, saying a new type of the H5N1 virus had rendered the vaccine ineffective.
"Based on post-vaccination surveillance results, a new clade of the H5N1 virus has appeared in Vietnam since 2010," the government said in a statement issued late on Thursday.
Vietnam had planned to use a vaccine imported from China for its vaccination this year, but the vaccine did not work against the new variety, the Animal Health Department said.
The new type has been circulating in northern provinces, coastal provinces in the central region as well as in the Central Highlands, it said.
The old type of the virus remains active in southern provinces, it said in a report this week.
Vietnam first detected the H5N1 virus in poultry in 2003. The virus has since infected 553 people in 15 countries and killed 323, most of them in Asia, including 59 in Vietnam, based on a World Health Organization tally.
Health experts fear the disease could mutate to a form that could be easily transmitted between humans. Almost all of the human H5N1 infections to date are believed to have passed from birds to humans.
Vietnam has detected no human cases of bird flu in 2011 but several provinces across the country have reported outbreaks in ducks and chickens.