Nathan Stein, 7, gets a H1N1 shots.

Alice Park
Time

As the number of H1N1 cases continues to climb in the U.S., researchers are collecting more and more data on the spread of the pandemic flu and getting a clearer picture of its victims — who is most vulnerable to H1N1, how the most severe cases progress and which risk factors tend to contribute to life-threatening disease. That data is now helping public-health officials identify some critical H1N1 trends, which may enable them to treat patients more effectively and hopefully control the disease as it peaks in the coming months.

The latest study, published this week in the Journal of the American Medical Association, offers a snapshot of 1,088 H1N1 cases in California that were severe enough to require hospitalization — or resulted in death — between April 23 and Aug. 11 of this year. Experts at the California Department of Public Health, who led the study, say their findings are largely in line with the growing body of data on the worldwide pandemic flu, confirming, for instance, that the 2009 H1N1 flu disproportionately affects younger patients. The California research team found that the median age of hospitalized H1N1 patients was 27, much lower than the median age of seasonal-flu sufferers.

While H1N1 infection results in mild or moderate disease in most patients — indeed, the most severe cases account for a small proportion of overall infections — a subset of patients are harder hit, the data show. And in those patients, the disease can often quickly become life-threatening. "The major point of our findings is that there has been a lot of perception that this is a mild disease, and a lot of people may be ambivalent about vaccination," says Dr. Janice Louie, a public-health medical officer at the California Department of Public Health and the study's lead author. "But for those patients who were hospitalized, 30% required intensive care. This is something that clinicians should be aware of when patients walk into their clinic or office with signs of flu."

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