Association between body mass index and laboratory-confirmed influenza in middle aged and older adults: a prospective cohort study

Background Studies conducted during the 2009 influenza A (H1N1) pandemic found that obesity increases the risk of severe influenza including hospitalization and death. In this study, we examined the relationship of BMI with having laboratory-confirmed seasonal influenza and influenza-related respiratory hospitalization. Methods We linked a cohort of 246,494 adults aged >= 45 years with data on BMI to subsequent laboratory-confirmed influenza notifications and cause-specific hospitalizations from 2006 to 2015. Cox-proportional hazard models were used to estimate the risk of incident laboratory-confirmed influenza and influenza-related respiratory hospitalizations according to BMI, adjusting for age, sex and other covariates. Results After 1,840,408 person-years of follow-up, 1891 participants had laboratory-confirmed influenza notifications (crude rate 10.3/10,000 person-years) of whom 623 were hospitalized for a respiratory illness. Compared to those with healthy BMI (22.5 to <25.0 kg/m(2)), influenza incidence was respectively 27% (adjusted HR [aHR]: 1.27, 95% CI: 1.10-1.46) and 69% (aHR: 1.69, 1.24-2.29) greater among obese (BMI: 30 to <40 kg/m(2)) and very obese adults (40 to <50 kg/m(2)). The equivalent aHRs for hospitalization were 1.57 (95% CI: 1.22-2.01) and 4.81 (95% CI: 3.23-7.17). For every 5-unit BMI increase above 22.5 kg/m(2), there was a 15% (aHR: 1.15, 95% CI: 1.09-1.22) increase in risk of having a diagnosis of influenza and 42% increase in hospitalization (aHR: 1.42, 95% CI: 1.30-1.60). These trends did not differ between the pandemic year (2009) and other years. Conclusions Our results suggest that obese adults have a similar risk of hospitalization for seasonal influenza as adults with cardiovascular disease and diabetes, and should therefore be equally prioritized for funded interventions such as targeted immunization programs.