A mathematical analysis has confirmed that H5N1 avian influenza spread from person to person in Indonesia in April, U.S. researchers reported on Tuesday.

They said they had developed a tool to run quick tests on disease outbreaks to see if dangerous epidemics or pandemics may be developing.

Health officials around the world agree that a pandemic of influenza is overdue, and they are most worried by the H5N1 strain of avian influenza that has been spreading through flocks from Asia to Africa.

It rarely passes to humans, but since 2003 it has infected 322 people and killed 195 of them.

Most have been infected directly by birds. But a few clusters of cases have been seen and officials worry most about the possibility that the virus has acquired the ability to pass easily and directly from one person to another. That would spark a pandemic.

Ira Longini and colleagues at the Fred Hutchinson Cancer Research Center in Seattle looked at two clusters -- one in which eight family members died in Sumatra in 2006, and another in Turkey in which eight people were infected and four died.

Experts were almost certain the Sumatra case was human-to-human transmission, but were eager to see more proof.

"We find statistical evidence of human-to-human transmission in Sumatra, but not in Turkey," they wrote in a report published in the journal Emerging Infectious Diseases.