We found obesity (BMI, ≥30) to be independently associated with an increased risk of respiratory hospitalizations during periods of seasonal influenza activity. The association between severe obesity (BMI, ≥35) and respiratory hospitalizations was present both for those without previously recognized risk factors for serious influenza complications and for those with 1 risk factor. In addition, severely obese individuals without risk factors had an event rate similar to that of normal weight individuals with only 1 risk factor, indicating that severe obesity may be as important a predictor of influenza complications as are other identified chronic conditions. We also observed a similar association using ICU admissions as an outcome. Although obese individuals are at increased risk of hospitalization from any cause during influenza season, the association was greater for respiratory conditions than for other conditions. Additional sensitivity analyses revealed specificity and consistency of the association.
This study examines the relationship between obesity and respiratory hospitalizations during seasonal influenza epidemics. Previous studies have revealed a high prevalence of obesity among individuals with complications of pandemic H1N1 infection [4
]. Furthermore, a recent case-cohort study reported that morbidly obese individuals (BMI, ≥40) had a 4–5 times greater risk of hospitalization for H1N1 than did normal weight individuals [30
]. Our study found a smaller association between severe obesity (BMI, ≥35) and hospitalization, and we detected an increase in risk for obese individuals in addition to morbidly obese individuals. Differences in the study populations, influenza strains, BMI categories, outcome measures, and study designs may explain some of the discrepancies.
This study had several limitations. BMI was based on self-reported height and weight, rather than direct measurement. Nevertheless, we showed that self-reported BMI reliably predicted measured BMI for this population. Any misclassification would be minor and, if present, would tend to underestimate our associations. Another limitation is that because individuals were included if they had completed a survey within 5 years of an influenza season, it is possible that their BMI may have changed over the study period. Data from the longitudinal NPHS suggests that BMI increases on average only 0.5 units over a 6-year period [31
]. Such a small increase would have resulted in minimal misclassification of individuals to BMI categories in the years following survey response in this study. The selected respiratory hospitalizations during periods of influenza circulation used as the outcome measure in this study were nonspecific and may have been due to causes other than influenza. However, they correlate well with influenza circulation and have been used in previous studies [22
]. We lacked sufficient event counts to adequately examine risk of outcomes in individuals without chronic conditions, as evidenced by the wider confidence intervals. Ontario’s universal influenza immunization program facilitates access to influenza vaccines through workplaces and community settings; thus, not all vaccination is captured by physician billing data [32
]. This may lead to misclassification of vaccination status. However, using self-reported vaccination as the gold standard, a physician billing claim for influenza vaccination had a positive predictive value of 0.89 and a negative predictive value of 0.81 for the age group in this study. Another limitation is that covariates derived from responses to the health surveys (smoking status and number of individuals living in household) may have changed over the study period and could not be updated at each index date. Finally, we were unable to distinguish whether obesity was associated with increased risk of influenza infection or an increased risk of serious complications arising from influenza infection.
Among the strengths of this study, the most notable was our ability to link population-based BMI data for a large number of individuals to health administrative data sets to determine outcomes, vaccination status, and comorbidities. This afforded a unique opportunity to study this association. Furthermore, we studied multiple influenza seasons, and we used influenza viral surveillance data to define periods of influenza activity. Future research should use laboratory-confirmed influenza outcomes and objective measures of height and weight, although it may not be feasible to collect such data on a sufficiently large population to examine serious complications from influenza.
Obesity is recognized as an important and growing public health problem because of its association with many serious chronic conditions. This study offers a new perspective on the dangers of obesity and its relationship to severe influenza infection. The results of this study suggest that severe obesity is associated with an increased risk of respiratory hospitalizations during influenza season, although this relationship may vary (ie, be stronger or weaker) in future years, with new influenza strains and different circumstances. To minimize this risk, severely obese individuals should be recommended to receive annual influenza vaccination and be considered for treatment with antiviral medications if they present with influenza-like illness during periods of influenza circulation. In the presence of universal influenza immunization, obese individuals should be prioritized to receive vaccine. Public health and clinical interventions to reduce the prevalence of obesity in the population may also reduce the annual health and economic burden of influenza.