A "striking" proportion of severe swine flu infections are occurring among young women, according to Canadian research that shows severe H1N1 can hit previously healthy teens and young adults hard and fast in a pattern previously only ever seen with 1918 Spanish flu. The study shows that severe disease and death in the outbreak is concentrated in relatively healthy young people, aged 10 to 60. 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 Monday, found an H1N1 death rate 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 striking 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 extraordinary "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. The Canadian study, by members of the Canadian Critical Care Trials Group, represents the largest series of patients with severe swine flu yet described, and includes both adults and children treated at 38 ICUs across Canada.
About one-third of critically ill patients in the Canadian study were aboriginal Canadians. Fifty of the 168 patients were children. As to why women are disproportionately hit, "nobody knows," Kumar said. In most infectious disease, males make up a lager proportion of cases, and have a higher death rate. Pregnancy is a risk factor for severe H1N1 disease. And women, on average, have less lung capacity than men. As well, obesity has been shown to be a risk factor for severe H1N1 disease. Excess fat tissue might further compromise a woman's lung capacity.