Severe obesity raises risk of poor H1N1 outcome

Severe obesity makes it almost five times more likely that adults infected with the H1N1 pandemic influenza virus will be ill enough to be hospitalized, and also more likely that they will die under certain conditions, according to new research by the Centers for Disease Control and Prevention (CDC).

The finding quantifies for the first time the frequent observation, made during clinical care and confirmed by surveillance data, that obesity has played a greater-than-expected role in H1N1 illnesses and deaths. Writing in the online medical journal Public Library of Science ONE (PLoS ONE), the researchers say that obesity appears to play such a powerful role that it exerts an effect independent of any underlying chronic conditions that patients may have.

The researchers were not able to say how obesity exerted that effect, but an unrelated piece of research may provide some insight. Using a study in mice, University of North Carolina researchers report in the Journal of Immunology that obesity caused by overfeeding seems to impair the immune system's ability to create the immunologic memory that allows the body to recognize and respond to flu infections.

The CDC study used data on hospitalizations and deaths gathered by state health departments early in the pandemic, between April and July 2009. As a control, the researchers compared the cases to a healthy cohort, using data from the National Health and Nutrition Examination Survey (NHANES), a nationwide representative survey.

For each group, the researchers calculated body mass index (BMI), a ratio of height to weight that is used as a universal measure of under- or overweight. They also recorded whether group members suffered from at least some of the chronic medical conditions—cardiovascular disease, pulmonary disease, liver disease, cancer, and diabetes—that federal health authorities have previously defined as playing a role in severe flu.

Pregnant women were not included in the study, even though they have also been observed to be at higher risk of H1N1 illness and death, because BMI-related definitions of overweight do not apply to them.

The results show that obesity appears to play a definite role in flu illness and death, though calculating the degree of its influence was limited by the quality of the data about some of the groups. Obesity had no effect on the illnesses and deaths of teenagers and children younger than 20, who were more likely to be very ill and die if they were underweight rather than overweight.

Among adults, morbid obesity—the highest category of overweight, defined as a BMI of 40 or more—played a significant role in the risk of developing a flu infection serious enough to require hospitalization. Those with chronic medical conditions were 4.9 times more likely to be hospitalized; those who did not have such conditions were 4.7 times more likely.

But the analysis of flu deaths did not show such a consistent effect of morbid obesity. In some runs of the data, the association between obesity and flu death was strongest for those without chronic underlying conditions; in other analyses, the strongest association was among those who died and did have chronic conditions. Oliver Morgan, PhD, a CDC epidemiologist and the first author of the study, said that lack of consistency was likely due to poorer-quality data on both BMI and underlying conditions among those who died, rather than to any lack of effect exerted by obesity.