The World Health Organisation on Friday said the virus that infected eight members of a family on the Indonesian island of Sumatra last month - killing seven of them - appears to have slightly mutated in a 10-year-old boy.
The boy is then suspected of passing the virus to his father, the WHO investigative report said.
However, the H5N1 virus, mainly transmitted through contact with infected birds, died with the father and did not pass outside the family, said Tim Uyeki, an epidemiologist from the US Centres for Disease Control and Prevention, following a bird flu conference in Jakarta.
In addition, Indonesian and WHO officials closely monitored more than 50 contacts of the victims in the case, keeping them in voluntary home quarantine following the outbreak, but none developed symptoms.
Uyeki added that as viruses are always changing slightly, there was no immediate cause for concern.
Scientists working to combat the virus regard any mutation leading to human-to-human transmission as as a step towards sparking a global human flu pandemic with a potential toll of millions.
The WHO estimates that there have been about half a dozen instances of human-to-human transmissions like the one in Indonesia, mainly involving relatives or carers who were infected by someone who caught the virus from poultry.
In each instance, the virus did not go beyond the immediate family or carers.
Bird flu has killed at least 130 people worldwide since it began ravaging Asian poultry stocks in late 2003. Vietnam has suffered the highest number of deaths with 42, and Indonesia is second on the list with 39.