After investigating the deaths of two sisters whose brother was also killed by the virus, the WHO said the two women had not contracted the virus from him.
Their deaths had been treated as the first possible case of human-to-human transmission of the disease.
But WHO in a statement said the test results on the second woman were reassuring and reflected the findings of tests carried out on the first sister last week.
"Virus genetic material from this woman, as for the other case in this cluster, is of avian origin and contains no human influenza genes," the statement said.
But despite the WHO assurance, European Union health ministers met in Brussels to take stock of the epidemic raging through much of Asia and considered measures if the virus mutated into human flu.
Earlier this month, WHO said human-to-human transmission was a "possible explanation" in the case of the Vietnamese women.
The WHO representative in Hanoi, Pascale Brudon said the test results did not exclude a very limited human-to-human transmission of the virus.
However she said "if any transmission took place, it did not spread outside this small cluster of cases, so it is of less concern."
The UN health agency has warned that the deadly H5N1 strain of bird flu could kill millions across the globe if it combines with a human influenza to create a new, highly contagious strain transmissible among humans.