Obesity and influenza associated mortality risks in an elderly cohort

Abstract

BACKGROUND: Obesity has never been identified as a risk factor for severe outcomes of influenza until the 2009 H1N1 pandemic. Few studies have explored the effect of obesity on influenza-associated mortality. OBJECTIVES: This study assessed the modifying effects of obesity on influenza-associated mortality risks using the data collected from a cohort of elderly patients. METHODS: A total of 66,820 elderly Chinese patients in Hong Kong, with a follow-up period of 1998-2009, were divided into 4 groups following the Asian adult criteria, according to their body mass indexes (BMI) at the baseline. These groups were underweight (BMI, <18.5), normal weight (BMI, 18.5-22.9), overweight (BMI, 23.0-24.9), obesity I (BMI, 25.0-29.9), and obesity II (BMI, ≥30). We adopted the Cox proportional model with time dependent covariates to estimate the excess risks of mortality associated with 10% increase of influenza activity (measured by weekly proportions of specimens positive for influenza). Modifying effects of obesity were assessed by interaction terms defined by products of influenza activity and obesity dummy variables, while other lifestyle factors and seasonal confounders were simultaneously adjusted for in the model. RESULTS: The interaction of obesity and influenza activity was found statistically significant in all-cause mortality (P < .05), but not in respiratory diseases, pneumonia, and influenza. Compared to the elderly patients with normal weight, underweight and obese elderly patients had higher mortality risks for these 3 disease categories, whereas overweight elderly patients had lower risks. Stratified analysis found that the hazard ratio of all-cause mortality associated with every 10% increase in influenza virus activity was 1.081 (95% CI, 1.013-1.154), 1.047 (95% CI, 1.012-1.084), 0.981 (95% CI, 0.936-1.028), 1.018 (95% CI, 0.980-1.058), and 1.062 (95% CI, 0.972-1.162) in the underweight, normal, overweight, obesity I, and obesity II groups, respectively. CONCLUSIONS: Both being underweight and obesity were associated with increased mortality risks attributable to influenza in Chinese elderly patients in Hong Kong