The findings -- published on a research-sharing platform, PLoS Currents: Influenza, vetted by flu experts -- could help health officials manage critical care resources if infection rates climb in the Northern Hemisphere's autumn and winter.

It could also provide important clues as to the new swine flu's potential virulence, said author Antoine Flahault, a leading epidemiologist and director of France's School for Advanced Studies in Public Health.

"The direct lethality due to viral pneumonia probably gives the best estimate of an influenza strain's virulence, since it may vary from strain to strain and is not due to a country's level of health development," he said.

With regular seasonal flu, which claims up to 500,000 lives each year worldwide, most deaths are attributed either to secondary bacterial infections such as pneumonia, or pre-existing chronic conditions that boost vulnerability.

Only about one-in-a-million infections result in death due to a rare condition known as acute respiratory disease syndrome (ARDS).

"ARDS is frighteningly lethal -- it is like drowning," Flahault told AFP by phone. The condition requires intensive-care treatment for an average of thee weeks. "Statistically, only one in two patients survive."

The risk factors for ADRS are poorly understood, though recent studies in the United States suggest that, among people infected with swine flu, pregnant women and the very obese are particularly at risk.

Both seasonal influenza and the new A(H1N1) virus that has swept the globe since May appear to have roughly the same mortality rate of one-to-five per 1,000 infections, though figures for the swine flu remain very sketchy.

Experts also caution that the pandemic flu could become more lethal as it continues to mutate.

But preliminary analysis of infection and mortality statistics from the French territory of New Caledonia and the Indian Ocean island nation of Mauritius suggest that the new A(H1N1) virus directly caused an ARDS fatality for every 10,000 cases, Flahault told AFP.

In New Caledonia, local health authorities have reported 30,000 infections and two deaths attributed directly to the pandemic virus.

Flahault said that some 70,000 persons have been infected in Mauritius, with seven reported deaths from ARDS, five of them confirmed.

"These surveillance data allow for a first estimate of direct lethality due to H1N1 of one-per-10,000 infections, about 100 times more than regular seasonal flu," he said.

Flahault cautioned that these are only "preliminary and rough estimates," and acknowledged that his findings -- based on a limited number of cases on two isolated islands -- may not extend to much larger, continental countries.

"But we have a terrible lack of data," he said.

"It may be useful to deliver such estimates ... as early as possible so health authorities can check availability of intensive care units and artificial ventilation devices in case of a wave of similar virulence this autumn in the northern hemisphere," he said.

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