The Mekong Delta province of Soc Trang has become the third province in Vietnam to report bird flu outbreak this year, the Department of Animal Health reported Sunday.
The agency made the statement just over two months after announcing that the epidemic had been curbed, along with foot-and-mouth disease in cattle.
According to the department, 90 chickens in Soc Trang's My Xuyen District have become ill or have died, adding that it has provided two million doses of the bird flu vaccine to the province in order to stop the recurrence from spreading.
Last week, central provinces of Thanh Hoa and Quang Tri also reported hundreds of cases of sick or dead fowl.
Health experts warned there is a high likelihood the disease will spread widely in the near future because the A/H5N1 virus in northern Vietnam has mutated and effective vaccines are not yet available.
Since the beginning of this year Vietnam has recorded two human fatalities caused by bird flu - an 18-year-old man in the Mekong Delta province of Kien Giang and a 26-year-old woman in Soc Trang Province, also in the Mekong Delta.
The cases occurred after nearly 20 months of no reported cases of humans contracting the A/H5N1 virus.
According to a World Bank report in October last year, Vietnam was among the countries most affected by H5N1, with 119 human cases confirmed as of March 2011, of which 59 were fatal.
Since 2003, bird flu outbreaks have killed and led to the forced culling of tens of millions of fowl in Vietnam.
The World Health Organization ranked Vietnam the country second most affected by bird flu after Indonesia.