Among adults with overweight or obesity, each-5 kg/m² increase in BMI is associated with a 40% increase in the risk for influenza-related hospitalization, a risk comparable to adults with cardiovascular disease or diabetes, according to findings reported in the International Journal of Obesity.
“Our results add to the growing evidence of a role of BMI in increasing the risk for diagnosed influenza and respiratory hospitalization not only in the pandemic year, but also in seasonal influenza years, particularly after 2009,” Surendra Karki, PhD, an epidemiologist with the School of Public Health and Community Medicine at the University of New South Wales in Sydney, and colleagues wrote. “To our knowledge, this is also the first prospective study to examine and report the association of laboratory-confirmed, seasonal influenza in relation to BMI.”
Karki and colleagues analyzed data from 246,494 adults aged at least 45 years randomly recruited for the Sax Institute’s 45 and Up study in New South Wales between 2006 and 2009 (mean age, 63 years; 53% women). Participant baseline data were linked to several databases to assess subsequent laboratory-confirmed influenza notifications and cause-specific hospitalizations from 2006 to 2015. Researchers used Cox proportional hazard models to estimate the risk for incident laboratory-confirmed influenza and influenza-related respiratory hospitalizations according to BMI, adjusted for age and sex.
Within the cohort, 39.6% had overweight and 21.8% had obesity. During 1,840,408 person-years of follow-up, researchers observed 1,919 laboratory-confirmed cases of influenza in 1,891 participants, with 185 of the cases during the 2009 influenza pandemic.
Compared with normal-weight adults, those with BMI between 30 kg/m² and 40 kg/m² were 27% more likely to receive an influenza diagnosis (HR = 1.27; 95% CI, 1.1-1.46), whereas adults with a BMI at least 40 kg/m² were 69% more likely to receive an influenza diagnosis (HR = 1.69; 95% CI, 1.24-2.29) with results persisting after adjustment for age, sex, income, smoking status and area of residence.
In adults with BMI of at least 22.5 kg/m², researchers found that for each 5-unit increase in BMI, risk for laboratory-confirmed influenza increased by 15% (HR = 1.15; 95% CI, 1.09-1.22), with no differences observed by age or sex.
Among participants with a laboratory-confirmed influenza diagnosis, 36% had at least one hospitalization event within 1 week and up to 2 weeks after the date of the influenza notification, according to researchers, for a total of 950 hospital admissions for 682 participants. Compared with normal-weight adults, the adjusted HR for influenza-related respiratory hospitalization was 1.57 (95% CI, 1.22-2.01) for those with BMI between 30 kg/m² and 40 kg/m², and 4.81 (95% CI, 3.23-7.17) for those with BMI between 40 kg/m² and 50 kg/m²
These trends did not differ between the pandemic year (2009) and other years, according to researchers.
“The results of our study should be interpreted carefully,” the researchers wrote. “We used influenza notifications to identify those infected. As up to 80% of those with influenza may not visit a health care facility, and only a fraction of those who do are likely to be tested, our rates of influenza notifications will underestimate the true rates.”
The researchers noted that the observed risk is comparable to that reported among adults with CVD and diabetes, suggesting adults with obesity should be equally prioritized for funding for targeted immunization programs to prevent severe influenza. – by Regina Schaffer
Disclosure: One of the study authors reports receiving funding or laboratory testing support for investigator-driven research from CSL Biotherapies, Merck and Pfizer, and serving on advisory boards for GlaxoSmithKline, Merck and Pfizer.