Study reveals obesity link to seasonal flu hospitalizations

Aug 16, 2011 (CIDRAP News) – A link between obesity and 2009 H1N1 flu complications was one of the surprising findings during the pandemic, and now a Canadian research group has uncovered a similar connection between obesity and respiratory hospitalizations during seasonal flu months.

During the pandemic the obesity link to complications led public health officials in North America and other countries to tweak vaccine priority planning, and the extension of the link to seasonal flu could help authorities design their policies to better protect this group.

The Toronto team's cohort study covered 12 flu seasons and was based on 82,545 adults ages 18 to 64 who responded to public health surveys. The findings appeared today in an early online edition of Clinical Infectious Diseases.

The flu seasons included 1996-97 through 2007-08. The surveys were part of a national cross-sectional effort by Statistics Canada, and the study group focused on Ontario, which had response rates ranging from 75% to 82%. The surveys were linked to health administrative data sets that allowed investigators to assess demographic factors and medical variables such as flu vaccination status and chronic medical conditions. The group defined two study periods: flu season and post flu season (control period).

Body mass index (BMI) was calculated from self-reported responses on the surveys. Categories included class 1 (BMI 30.0 to 34.9), class 2 (35.0 to 39.9), and class 3 (40 or more). The number of respondents in class 2 and 3 categories were small, so the researchers combined the two groups.

About 12% of the group had BMIs in the class 1 obesity category, and 5% were in the class 2 and 3 tiers.

The outcome measure was hospitalization for pneumonia and influenza.

Researchers found that obesity was independently associated with an increased risk of respiratory hospitalization during flu seasons, a risk that was present even for severely obese patients with no underlying medical conditions. The odds ratio for the class 1 group was 1.45 [95% confidence interval (CI), 1.03-2.05] and 2.12 for the class 2 and 3 groups [95% CI, 1.45-3.10].

The risk for the most obese group with no chronic medical conditions was about the same as for normal-weight people who had one risk factor for flu complications, which the authors wrote indicates that severe obesity may be as important a predictor of flu complications as other previously identified underlying medical conditions. "We also observed a similar association using ICU admissions as an outcome," they added.

"This study offers a new perspective on the dangers of obesity and its relationship to severe influenza infection," the research group stated, adding that people in obese categories should be advised to receive annual flu vaccinations and be treated with antiviral medication if they get sick with flu-like symptoms when the viruses are circulating.

In an editorial on the study findings, which appears in the same journal issue, two influenza experts from the US Centers for Disease Control and Prevention (CDC) wrote that the group's findings are strengthened by their sensitivity analysis showing that during influenza seasons, obese people had increased odds for all hospitalizations except those due to external injury. Dr. Seema Jain and Dr. Sandra Chaves, with the CDC's National Center for Immunization and Respiratory Diseases, wrote, "Importantly, [the researchers] did not find an association between obesity and risk of hospitalization due to respiratory illness during non-influenza seasons."

They also lauded the way the Canadian group linked the electronic administrative data sets to national surveys, but Jain and Chaves said technique might not tease out correlations of conditions such as diabetes with obesity, because the condition often goes undiagnosed, which could lead to an overestimation of obesity as an independent risk factor.

Though they wrote that other studies suggested a connection between obesity and infections, including pneumonia, even before the 2009 H1N1 pandemic, the Canadian group is the first to investigate the link between obesity and seasonal flu, the two wrote.

The growing body of evidence about obesity as a flu complication risk is already helping guide flu vaccine recommendations, but further studies of the link are warranted to help guide prioritization planning, especially when resources are limited, according to the authors.

Kwong JC, Campitelli MA, Rosella LC. Obesity and respiratory hospitalizations during influenza seasons in Ontario, Canada: a cohort study. Clin Infect Dis 2011 Sep;53(5):413-21 [Full text]

Jain S, Chaves SS. Obesity and influenza. (Editorial) Clin Infect Dis 2011 Sep;53(5):422-4 [Full text]

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