People who die after getting swine flu are 100 times more likely, compared to seasonal flu, to have been killed by the virus itself rather than secondary causes, a top French researcher said Monday.

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 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."

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.