Early tests on an Indonesian family that is being investigated for possible person-to-person transmission of bird flu do not suggest the virus has changed significantly, says the World Health Organization.
The tests looked at the genetic make-up, or 'sequence', of the virus that infected the family members who fell ill first.
A spokesperson from the World Health Organization (WHO) told SciDev.Net that researchers are looking at sequences of the viruses found in the relatives who became ill later.
The family, from North Sumatra, has sparked interest among influenza experts because it is the largest family group known to be infected by the H5N1 virus so far. Six of seven family members who have died were confirmed to be infected with the virus, and an eighth relative is being treated in hospital.
Another unusual aspect of the case is that no one has been able to determine when or if the eight relatives came into contact with sick birds.
The uncertainty has led to investigations into the possibility that the virus could have been transmitted between them.
According to the WHO, "All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness."
The WHO spokesperson said no other villagers had shown any flu-like symptoms, although some have been quarantined.
She also said the WHO is investigating a second bird flu cluster in a brother and sister who died earlier this week in Bandung, West Java. A laboratory in Hong Kong is verifying that they were suffering from H5N1 infection.
Researchers at the Hong Kong laboratory are also continuing to sequence viral samples from the Sumatra cluster. Results are expected next week.
Possible cases of human-to-human transmission have been seen before, though never on this scale (see Bird flu deaths raise fears of human spread).