Body mass index and the incidence of influenza-associated pneumonia in a UK primary care cohort

Background Accumulating data suggest an association between increased BMI/obesity and morbidity in patients with pandemic (H1N1) 2009 influenza. Information on metabolic status and prognosis in seasonal influenza is lacking, however. Methods A retrospective cohort study was carried out using the UK General Practice Research Database. Patients aged 18 with 1 recorded BMI in the 12-58 kg/m(2) range between January 1, 2000, and December 31, 2007, were observed for an influenza-associated pneumonia diagnosis after the date of baseline BMI, including 'influenza with pneumonia' or a diagnosis of 'pneumonia' up to 30 days after a diagnosis of 'influenza'. Results A total of 1 074 315 patients were included, of whom 73.2% were within the reference BMI range or overweight and 2.2% were underweight (<18.5 kg/m(2)). Pneumonia rates were 32.33-37.48/100 000 in all BMI categories except the underweight (98.29/100 000). Relative to patients with acceptable weight, those who were underweight had an increased pneumonia rate [adjusted IRR = 2.32 (95% CI 1.80-2.94)], while being overweight (BMI = 25.0-29.9 kg/m(2)) or obese (BMI 30.0 kg/m(2)) was associated with a decreased pneumonia rate [adjusted IRR = 0 77 (95% CI 0.68-0.86) and 0 85 (95% CI 0.73-1.00), respectively]. On the other hand, women and obese women with type 2 diabetes had increased pneumonia rates [adjusted IRR = 1 37 (95% CI 1.08-1.72) and 1 47 (95% CI 1.01-2.06), respectively]. Conclusions In contrast to initial data from pandemic influenza, influenza pneumonia, and pneumonia following influenza were the most common in underweight persons, and an apparent decreased rate of pneumonia was noted with increasing BMI categories. Women with type 2 diabetes had increased rates of pneumonia.