The pandemic flu bug is far more likely to strike younger people. But when people aged 50 and older get hospitalized with H1N1 swine flu, their case-fatality rate is the highest of any group: 18% to 20%.
The findings come from an analysis of data collected from California hospitals during the first 16 weeks of the U.S. H1N1 swine flu pandemic (April 23 to Aug. 11) by Janice K. Louie, MD, MPH, of the California Department of Health and colleagues.
"In contrast with the common perception that pandemic 2009 influenza A (H1N1) causes only mild disease, hospitalization and death occurred at all ages, and up to 30% of hospitalized cases were severely ill," Louie and colleagues report in the current issue of the Journal of the American Medical Association.
In a news conference, CDC Director Thomas Frieden, MD, MPH, said the California data show that H1N1 swine flu is just as deadly as seasonal flu.
"What we have seen in that article and in our own data from around the country and around the world is the level of severity among those who become ill is similar to seasonal flu," Frieden said. "Although a much, much lower proportion of people over 65 get H1N1 compared with seasonal flu, if they get it, it can be every bit as severe."
Louie and colleagues note that the low median age of patients with severe or fatal H1N1 swine flu -- 27 years -- makes the pandemic flu "markedly different" from seasonal flu.
"A striking percentage of hospitalized cases were severely ill, with more than 30% requiring intensive care; most adults and more than one-third of children required mechanical ventilation," they write. "Eleven percent died; the most common reported causes of death were viral pneumonia and acute respiratory distress syndrome."
And as previous data have shown, pregnant women are at much higher risk of severe flu than are other healthy women.
"Of note, 20% of hospitalized pregnant women in our series required intensive care; most were in their second or third trimester of pregnancy," Louie and colleagues report. They note that similar observations were made in the flu pandemics of 1918-1919 and 1957-1958.
Obesity a Risk for Severe H1N1 Swine Flu
As others have seen, more than 70% of adults and more than 60% of children with severe H1N1 swine flu had underlying medical conditions.
The California data add to a growing body of evidence that extreme obesity -- a body mass index or BMI of 40 or more -- is a risk factor for severe swine flu. Nearly half of all the severe cases in California were in obese people; 43% had a BMI of 40 or more.
Louie and colleagues note that nearly a third of obese Californians with severe swine flu did not have an established risk factor, although many had other conditions such as high blood pressure.
"A link between obesity and severe influenza, while not proven, is plausible," they suggest.
The CDC's Frieden agrees that obesity may well turn out to be an independent risk factor.
"We are in the midst of an epidemic of obesity. It has doubled in adults and tripled in children in the past couple of decades," Frieden said. "We are still trying to understand what all of the implications of that are for people's health. Increased susceptibility to infection is one, reduced respiratory reserve and ability to fight off infections is another, but this is something we need to learn that more about."