Citing a report from Vietnam's health ministry, the United Nations agency said the woman from Dam Ha district in Quang Ninh province developed symptoms on Jan. 28, and was sent to hospital on Jan. 31.
She is known to have had contact with sick and dead poultry before falling ill, the WHO said.
Of 108 cases of human bird flu in Vietnam confirmed so far, 52 have been fatal. Worldwide, 407 human infections have been confirmed since 2003, of which 254 have been fatal, WHO said.
Avian influenza has affected millions of birds around the world, particularly in East Asia, and can be caught by humans who come into contact with infected birds -- an ever-present danger in some societies where people keep poultry in the yards of their homes or live close to food markets.
Scientists fear the H5N1 strain of bird flu virus could mutate into a form that would spread easily between people, leading to a pandemic that could infect millions.
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