A "striking" proportion of severe swine flu infections are occurring among young women, according to Canadian research.

The research has found that severe H1N1 can hit previously healthy teens and young adults hard and fast in a pattern previously only seen with the 1918 Spanish flu.

Severe disease and death in the outbreak is concentrated in relatively healthy people, ages 10 to 60, the study found. Only 30 per cent had serious underlying health problems, such as cancer, chronic kidney failure or medication-dependent diabetes.

"Seventy per cent of them would have said to you, 'I'm pretty healthy.' I think that's pretty startling, quite frankly," said Dr. Anand Kumar, an intensivist with the Winnipeg Regional Health Authority and an associate professor of critical care and infectious disease at the University of Manitoba.

The study found that of the 113 women and 55 men admitted to an intensive care unit between April and August, the mean age was 32. Overall, 29 people (17 per cent) died -- most within the first 14 days after becoming critically sick. Twenty-one (72 per cent) of those who died were female.

"It's a high mortality rate for young people," Kumar said. "When you bear in mind that these are people from the age of 15 to 55 years old, you don't see a lot of diseases that have that kind of mortality in a large group of people that age. These people are people in the prime of their lives."

Another study, also released yesterday, found an H1N1 death rate more than twice as high in Mexico. By 60 days, 24 of the 58 patients (41 per cent) admitted to intensive care with H1N1 in the Mexican study had died.

The major difference between Mexico and Canada is access to high-tech care, Kumar said.

But there were similarities in the findings from both countries, investigators said. Once admitted to hospital, people with severe H1N1 rapidly got worse, suffering severe hypoxemia -- lower than normal oxygen in the blood -- that required, on average, 12 days of mechanical ventilation and "rescue" therapies to keep them alive. One such therapy is a machine that takes blood out of the body, oxygenates it and returns it to the body, similar to a bypass machine for heart surgery.

"Unlike [heart] surgery, when you do this for a couple of hours, some of these guys required it for a week or two weeks, or longer," said Kumar. "Rescue therapies are therapies that we use when we've got nothing else left," he said. They are also not widely available in Canada.

Shock and multiple organ failure were also common in the sickest patients.

As to why women are disproportionately hit, "nobody knows," Kumar said. In most infectious disease, males make up a larger proportion of cases, and have a higher death rate.

The study, by the Canadian Critical Care Trials Group, included adults and children treated at 38 hospitals across Canada.