A new analysis has confirmed that bird flu spread from person to person in Indonesia in April, U.S. researchers report in what appears to be a disturbing development for the infectious disease.

Health officials around the world have been closely monitoring the H5N1 strain of avian influenza spreading among birds from Asia to Africa to Europe.

So far, the strain rarely infects humans. But infectious disease experts are worried if it evolves so that it can spread easily from person to person, it may be the source of the next influenza pandemic, for which the globe is thought to be well overdue.

Since 2003, H5N1 has infected 322 people and killed 195. Most have been infected directly by birds. But a few clusters of cases have been noted for which no other explanation can be found except person-to-person transmission.

Biostatistician Ira Longini and colleagues at the Fred Hutchinson Cancer Research Center in Seattle looked at two such recent 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 cases were human-to-human transmission, but were eager to see more proof. Longini's team claims they have found that proof, reporting in the journal Emerging Infectious Diseases.

Using a computerized disease-transmission model that took into account the number of infected cases, the number of people potentially exposed, the viral-incubation period and other parameters, the researchers produced the first statistical confirmation of humans contracting the disease from each other.

How the cluster likely spread

The cluster likely began with a 37-year-old woman, who had been exposed to dead poultry and chicken feces, the presumed source of infection. She then probably passed the virus to her 10-year-old nephew who then passed it to his father.

The possibility that the boy infected his father was supported by genetic sequencing data. Other person-to-person transmissions in the cluster were backed up with statistical data.

All but one of the flu victims died.

Local health authorities eventually placed more than 50 relatives and close contacts under voluntary quarantine and the infections stopped. But Longini's team does not believe the quarantine did the trick; they believe the virus simply burned out.

"It went two generations and then just stopped, but it could have gotten out of control," Longini said in a statement.

"The world really may have dodged a bullet with that one, and the next time, we might not be so lucky."

The researchers now estimate the secondary-attack rate, which is the risk that one person will infect another, is at about 29 per cent. This is similar to what is seen for regular, seasonal influenza A in the United States.

As for the cluster in Turkey, Longini's team could not find statistical evidence of human-to-human transmission.

"There probably was person-to-person spread there as well but we couldn't get all the information we needed for the analysis," biostatistician Yang Yang said.

Longini's team also says they have developed a tool to run quick tests on disease outbreaks to see if dangerous epidemics or pandemics may be developing. The software product, called TranStat, would be available free of charge on the National Institutes of Health's Models of Infectious Disease Agent Study, or MIDAS, website.