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."
Among hospitalized patients in the study, 118 died an overall 11% fatality rate. Although the rate of hospitalization was highest among infants under 2 months old, the rate of death was highest in patients over age 50; H1N1 was least likely to turn fatal in patients under age 17. Yet with all the focus in the media on the vulnerability of younger patients to infection, the elderly may have been somewhat dangerously overlooked, says Louie. Although older patients may not be at high risk of getting infected in the first place (thanks to their residual immunity to the virus from previous outbreaks of H1N1), their risk of death from the disease may be higher than that of younger patients, primarily because of their higher rates of underlying conditions, such as heart disease, reduced lung function, diabetes and emphysema.
Indeed, the state's data suggest that chronic underlying conditions are among the main risk factors for developing H1N1 disease severe enough to require hospital care. In both young and old patients who were hospitalized for swine flu (741 cases in total), ailments that complicate the flu were common: some 60% of children and 72% of adults had conditions including high blood pressure, high cholesterol and gastrointestinal disease.
The California data also reveal a potentially new risk factor for H1N1: obesity. Obese individuals were disproportionately represented in the state's sample of hospitalized cases 58% of adults age 20 or older registered as obese, and 43% of these morbidly so. Those with excessive body-mass-index measurements tend to have other medical conditions related to weight that may put them at risk of suffering more severe infection with H1N1. Being overweight can increase sleep apnea and reduce lung function, for example, both of which can impair a heavier person's ability to overcome a respiratory infection like influenza. Among the 156 obese adults in Louie's sample, 66% had underlying diseases known to complicate the flu, including chronic lung disease, heart disease and diabetes.
But what's baffling, say the study's authors, is that obesity does not stand out as a risk factor in severe cases of seasonal influenza. "A lot of us are puzzling over this, because this is not a trend with seasonal influenza in the limited studies that have been done in that area," says Louie. "It may be that H1N1 does cause more aggressive viral pneumonia, and some pathologic studies suggest this [H1N1] virus does have an affinity for receptors in the lower lung, but nobody really knows."
"Obesity has doubled in adults and tripled in children over the past couple of decades," Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, said in response to the study during a briefing with reporters. "We still need to understand what the consequences of that are. Increased susceptibility to infection is one. Reduced respiratory reserve is another. But it's something we still need to learn more about."
For now, Louie urges continued vigilance as flu season wears on. "This is not in my mind a disease to be taken lightly at all," she says. "Especially if you have risk factors."