The World Health Organisation has moved quickly to quell fears that the H5N1 strain of bird flu may have mutated into a form more likely to cause a pandemic in humans.

The news that human-to-human transmission of the virus may have occurred in an Indonesian family raised concerns that the virus had mutated into a more easily-transmissible form, but the WHO said the cluster of infections could have resulted from ‘close and prolonged exposure to a patient during a phase of severe illness’, in other words a very high inoculum of the virus.

“There is no evidence that efficient human-to-human transmission has occurred,” said the WHO in a statement, pointing out that it is also possible that the patients were all exposed to infected birds.

Initial testing of the virus has also revealed no signs of it mutating or recombining, it said. It is generally thought that H5N1 would have to undergo quite a major genetic mutation in order to spread easily between people and trigger a pandemic.

So far, bird flu has killed 124 people in 10 countries, most of them in Vietnam and Indonesia, out of a total of 218 confirmed cases of the infection.