20 January 2012
Body mass index and the incidence of influenza associated pneumonia in a United Kingdom primary care cohort
The January 2012 issue of Influenza and Other Respiratory Viruses contains a report by Blumentals et al. which examines the relationship between body mass index (BMI) and the risk of pneumonia related to influenza. The retrospective cohort study examined the association between BMI and the incidence of seasonal influenza-associated pneumonia in a primary care cohort in the United Kingdom (UK). The study included more than 1 million UK general practice research database patients ≥ 18 years of age with a recorded BMI in the 12-58 kg/ m2 range who were reported to have had a diagnosis of "influenza with pneumonia" or "pneumonia" within 30 days of a diagnosis of "influenza". The authors included data from an eight year period from January 1, 2000 and December 31 2007. The investigators found that incidence rates of influenza-associated pneumonia increased with age, however, in contrast to several previous studies examining metabolic state and influenza severity, this study found an inverse J-shaped relationship between BMI and pneumonia incidence with the highest rates of influenza-associated pneumonia occurring in the lowest BMI stratum (<18.5kg/m2). In that group, the pneumonia incidence was 98.29 per 100,000 person-years versus 32.33 per 100,000 person-years in persons with BMI >30 kg/ m2. In addition, while the presence of type 2 diabetes was associated with twice the incidence of pneumonia when comparing diabetic and non-diabetic patients in the same BMI group, the disease was only weakly associated with increased risk of pneumonia when controlling for other risk conditions and comparing to the risk in individuals with BMI of 18.5 - 24.9 without diabetes. The authors did a further subgroup analysis on individuals aged 18 - 49 years and found that BMI and type 2 diabetes were not independent risk factors for pneumonia. Notably, smoking was also found to be associated with increased rates of pneumonia. The authors conclude that the association between BMI status and outcomes of influenza infection remains unclear and requires further study because of its public health implications.
The study by Blumentals et al. provides intriguing new information about the association between body weight and risk of severe influenza disease. It is important to note that in contrast to much of the data that has been published related to the pandemic that this study was not limited to laboratory confirmed influenza cases and was covered a period of time during which influenza A(H3N2) was by and large the predominant influenza virus circulating in the country. The authors note that pandemic influenza A(H1N1) which appeared in 2009 affected a different age group than that generally affected by seasonal viruses, particularly A(H3N2). They attempt to address this issue by doing a sub analysis on the same age group that was most affected by the pandemic virus and found that obesity was still not a risk factor for severe disease. Although there are many potential explanations for why obesity, or even low body weight, might increase risk of severe disease related to influenza, other studies of obesity and the risk of severe respiratory disease have also found inconsistent results. A Canadian cohort study over 12 influenza seasons found that patients with BMI > 30 without any other risk factors were more likely to have a respiratory hospitalization during an influenza season than normal weight individuals. A multi year Danish study found a strong association between obesity and pneumonia hospitalization but the association disappeared when adjusted for major chronic diseases, though this study did not specifically evaluate influenza pneumonia. However, in a large systematic review and meta-analysis of pandemic 2009 studies conducted by Fezeu et al, the authors concluded that while obesity was not associated with increased risk in every study reviewed, overall obesity doubles the risk of ICU admission or death in patients hospitalized with pandemic influenza A(H1N1) infection. The study was limited by the lack of data to evaluate confounding factors such as co-morbid conditions and prolonged length of hospital stay but the association was even stronger in the morbidly obese. Until this important question is answered, influenza patients with abnormal body weight, particularly those with associated comorbid conditions, should be watched and managed carefully.
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