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H1N1 Swine Flu Deadly in All Age Groups

When Swine Flu Is Bad, It's Really Bad, Data Confirm
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WebMD Health News
Reviewed by Louise Chang, MD

Nov. 3, 2009 - H1N1 swine flu isn't always severe, but when it's bad, it's really bad. Patients hospitalized with pandemic flu have an 11% fatality rate, data from California suggest.

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.

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