More than half the fatalities from swine flu have been among young adults, according to one of the first surveys to gather mortality data from across the globe for the new A(H1N1) virus.
The analysis of 574 pandemic deaths from 28 countries through mid-July, released this week, also found that being diabetic or obese significantly boosted the risk of dying.
Neither children nor the elderly are as vulnerable as initial reports indicated, found the study, published by Eurosurveillance, the monitoring arm of the European Centre for Disease Prevention and Control.
"Most deaths (51 percent) occurred in the age group of 20-to-49 year-olds, but there is considerable variation depending on country or continent," the researchers reported.
Only 12 percent of those who died were 60 or older.
All of these features -- high mortality among young adults and the obese, but not the very young or elderly -- are sharply different than for the seasonal flu.
More than 90 percent of deaths from seasonal flu -- which claims 250,000 to 500,000 lives annually according to the WHO -- are in people over 65.
By contrast, with the pandemic H1N1, "the elderly seem to be protected from infection to some extent, perhaps due to previous exposure to similar strains," the study conjectured.
Persons born before 1957, other studies have suggested, were almost certainly exposed to the milder seasonal A(H1N1) viruses that evolved from the terrible pandemic of 1918, which left some 40 million dead.
With the 2009 strain, "when infection does occur, however, the percentage of deaths in elderly cases seems to be higher that in others."
One common target across both pandemic and season strains is pregnant women, according to the study, led by Philippe Barboza of the French Institute for Public Health Surveillance.
On Tuesday, the European Union said pregnant women should have priority in the distribution of vaccines, along with health workers and people with underlying health problems.
The data underlying the study also suggests that about six people die for every 1,000 infections, two or three times the rate of seasonal flu, but far less than the deadly pandemic of 1918.
The researchers caution, however, that it is far too early to calculate the "case-fatality ratio" (CFR) with much accuracy.
"Evaluating CFR during a pandemic is a hazardous exercise. Aside from the issue of whether or not a death has been caused by the influenza infection, cases tend to be detected initially among severely ill patients with a higher probability of dying," they conclude.
This leads to an over estimation of how lethal a virus is, they note.
Swine flu first erupted in Mexico in April, and has since swept across the globe, infecting hundreds of thousands and leaving more than 1,800 dead, according to the World Health Organisation.