An analysis of more than 1,000 California patients hospitalized with H1N1 flu during the first four months of the pandemic found that infants were most likely to be admitted, and patients 50 and older were most likely to die once admitted.
In the first four months of the pandemic, H1N1, like the seasonal flu, was especially severe in older people, who are more likely to have underlying health conditions, says lead author Janice Louie, a public-health medical officer at the California Department of Public Health.
However, Louie says, unlike seasonal flu, older people are far less likely than children and young adults to contract the H1N1 flu in the first place. For that reason, the study won't lead the Centers for Disease Control and Prevention to add healthy older people to the list of priority groups for H1N1 vaccine, director Thomas Frieden told reporters Tuesday.
Of 1,088 patients hospitalized with H1N1 flu in California, 11%, or 118 patients, died, and 30%, or 340 patients, were admitted to intensive-care units, Louie and her co-authors report in today's Journal of the American Medical Association. In patients 50 and older, the death rate was up to 20%, compared with about 2% in hospitalized patients under age 18.
The study focuses on patients who were hospitalized between April 23 and Aug. 11. Whether H1N1, or swine flu, will eventually mutate and cause more severe illness is not yet known, Louie says: "Influenza is pretty unpredictable."
Nearly a third of all the hospitalized patients in her study were reported to have no underlying conditions, such as lung disease, associated with an increased risk of flu complications.
But a disproportionate number of them were obese, an observation that also has been made in other countries, the authors write. Obesity doesn't appear to be a risk factor for seasonal flu.
Of the 361 patients whose body mass index – or BMI, a number based on height and weight – was known, half were obese, and half of those patients were morbidly obese, defined as having a BMI over 39, or roughly 100 pounds overweight.
Like the hospitalized patients overall, nearly a third of the obese patients were reported to have no known risk factor for severe influenza, although about a quarter had other health conditions, such as high blood pressure.
Because an increased risk of severe flu in obese patients has been observed in several locations, "I think it's probably real," says Gordon Rubenfeld, a critical-care specialist at the University of Toronto. Why, Rubenfeld says, remains to be seen.
Ali El Solh, an associate professor of medicine and social and preventive medicine at the State University of New York-Buffalo, speculates that obesity-related inflammation might further damage flu patients' lungs. In Louie's study, the most common causes of death were viral pneumonia and acute respiratory distress syndrome.
Contributing: Steve Sternberg