Maria Cheng, a spokeswoman for the U.N. agency, was quoted as saying its determination was based on its investigation of a recent cluster of cases in northern Sumatra, where seven members of a single family were killed in May.
"There was a mutation found, it was in a report recently given to the (Indonesian) government. It was the summary of the investigation into the northern Sumatra case," she told reporters. "But it did not mutate into a form that is more transmissible because it didn't seem to go beyond the cluster."
Indonesian and WHO officials closely monitored more than 50 contacts of the victims, keeping them in voluntary home quarantine for several weeks following the outbreak, but none developed symptoms, the agency reported.