The World Health Organisation (WHO) has not ruled out the possibility of Bird flu spreading through human contact.
The UN health monitoring body last Friday said tests on victims of the disease had shown no genetic evidence that bird flu could be passed from person to person.
But today it reversed the finding saying it had insufficient evidence to make the claim categorically.
Last Friday's announcement had been greeted with huge relief because of fears that the deadly virus, which has prompted the culling of millions of chickens and other fowl across Asia, could spread by humans.
But the WHO said that there had been a mix-up in the testing of two Vietnamese sisters who died from the killer virus.
The WHO originally said it had received the results of tests from both sisters but today said the findings from one of them had not been returned.
"She was the one who apparently had had no direct contact with any diseased bird," said WHO spokesman Mr Dick Thompson. "So we cannot rule out human-to-human transmission," he added.