Obesity and 2009 pandemic influenza A(H1N1) – its role and implications as an important risk factor for the development of severe influenza diseaseArchived

ECDC comment

2 papers are reviewed: A Novel Risk Factor for a Novel Virus: Obesity and 2009 Pandemic Influenza A (H1N1) and Morbid Obesity as a Risk Factor for Hospitalization and Death Due to 2009 Pandemic Influenza A(H1N1) Disease.

A Novel Risk Factor for a Novel Virus: Obesity and 2009 Pandemic Influenza A (H1N1)

Louie JK, Acosta M, Samuel MC, et al.  Clin Infect Dis. (2011); first published online: January 4, 2011

This paper describes a study conducted in the United States (US) which investigates the role of obesity as an independent risk factor for severe infection with 2009 pandemic influenza A(H1N1), trying to elucidate its relative weight among the different co-morbidities with which obesity is normally associated. The authors of the study used public health surveillance data to analyse demographic and clinical characteristics of residents in California, US, hospitalized with influenza A(H1N1)2009 infection with the purpose of assessing whether obesity (body mass index [BMI] ≥30) and extreme obesity (BMI ≥40) were an independent risk factor for death among case patients ≥ 20 years old. The study includes observations from 534 adult case patients infected with influenza A(H1N1)2009, and for whom BMI information was available, during the period 20th April to 11th August 2009. Out of the total number of patients, 228 (43%) were ≥50 years of age and 378 (72%) had influenza-related high-risk conditions recognised by the Advisory Committee on Immunization Practices (ACIP) as risk factors for severe influenza. In addition, 274 out of 534 (51%) had BMI ≥30, which is 2.2 times the prevalence of obesity among California adults (23%) and 1.5 times the prevalence among the general population of the United States (33%). Of the 92 case patients who died (17%), 56 (61%) had BMI ≥30 and 28 (30%) had BMI ≥40. The authors performed the statistical analysis of the data and the multivariate analysis showed that BMI ≥40 (odds ratio [OR], 2.8), BMI ≥45 (OR, 4.2), age ≥50 years (OR, 2.1), miscellaneous immunosuppressive conditions (OR, 3.9) and asthma (OR, 0.5) were associated with death. The highest of these odds ratios was that associated with obesity, demonstrating the association of this condition with increased odds of death.

Morbid Obesity as a Risk Factor for Hospitalization and Death Due to 2009 Pandemic Influenza A(H1N1) Disease

Morgan OW, Bramley A, Fowlkes A, et al.PLoS ONE 5(3): e9694; 2010

This authors of this study, carried out also in the US, investigated whether obesity is an independent risk factor for hospitalization and death due to 2009 pandemic influenza A(H1N1), against the alternative hypothesis that obesity is associated with chronic medical conditions considered by the ACIP to increase the risk of influenza-related complications. They used a case-cohort design to compare cases of hospitalizations and deaths from 2009 pandemic A(H1N1) influenza occurring between April and July, 2009, with a cohort of the US population estimated from a survey performed from 2003 to 2006 – the National Health and Nutrition Examination Survey (NHANES). The authors defined categories of relative weight by body mass index (BMI, kg/m2) in the case of hospitalizations. Information pertaining to death patients with obesity/morbid obesity was recorded on medical charts and death certificates. The study included 361 hospitalizations and 233 deaths with which it was possible to determine BMI category and presence of ACIP-recognised chronic conditions. Among ≥20 year olds, hospitalization was equally associated with being morbidly obese (BMI≥40) between individuals with ACIP-recognised chronic conditions (OR = 4.9) and without ACIP-recognised chronic conditions (OR = 4.7). Among 2–19 year olds, hospitalization was more associated with being underweight (BMI≤5th percentile) among those with ACIP-recognised chronic conditions (OR = 12.5) than among those without ACIP-recognised chronic conditions (OR = 5.5). In addition, the results show that, although death was not associated with BMI category among individuals 2–19 years old, among individuals aged ≥20 years without ACIP-recognised chronic conditions death was associated with both obesity (OR = 3.1) and morbid obesity (OR = 7.6). In conclusion, this study revealed that morbidly obese people were at a higher risk of hospitalization due to influenza A(H1N1)2009 infection than people with normal weight. Data from this study also suggest that the risk of death following influenza A( H1N1) infection may have been higher for morbidly obese individuals.

ECDC Comment (25th March 2011):

During the 2009 A(H1N1) influenza pandemic, early reports from the US [1] and abroad suggested that obesity was more frequent among people hospitalized with influenza A(H1N1)2009 infection or who died because of it. One of the earliest studies that attempted to compare the prevalence of risk factors among severely ill pandemic influenza cases with the prevalence of the same risk factors in the general population was conducted among all confirmed 2009 pandemic influenza A(H1N1) cases requiring Intensive Care Unit (ICU) admission in Australia and New Zealand [2]. It showed a series of risk factors which were much more common among cases with confirmed severe influenza disease than among the general population: pregnant women (9% versus 1%), obesity [defined as BMI greater than 35] (29% versus 5%) and asthma or other chronic pulmonary disease (33% versus 13%).

All these studies, including the two described above and the data from Australia, New Zealand and the US, show that obese and morbidly obese people were at a higher risk of developing severe disease and/or dying due to influenza A(H1N1)2009 infection, even without any other previously recognised high risk conditions. The latest ECDC’s 2009 influenza A(H1N1) pandemic Risk Assessment [3] elaborates on and explains these findings in detail. Additionally, the ECDC’s 2009 Pandemic Forward Look Risk Assessment [4] suggests that a retrospective review of the risk groups in EU Member States should be carried out through surveillance data on severe acute respiratory infection and deaths with the aim of revisiting the previous recommendations on risk and target groups. In this sense, special attention may need to be paid to the ‘new’ risk groups, notably young children, pregnant women and those with morbid obesity. Based on the above considerations, obese and morbidly obese individuals should get vaccinated and should be treated early with antivirals if they do become sick with influenza. Morbid obesity, with or without the presence of other underlying high risk health conditions, is associated with a higher risk of 2009 pandemic influenza A(H1N1)-related hospitalization and possibly death.


  1. 2009 H1N1 Early Outbreak and Disease Characteristics. October 27, 2009, 6:00 PM ET, US Centers for Disease Control and Prevention (CDC).
  2. Baker MG, Wilson N, Huang QS, Paine S, Lopez L, Bandaranayake D, et al. Pandemic influenza A(H1N1)v in New Zealand: the experience from April to August 2009. Euro Surveill. 2009;14(34).
  3. Pandemic risk assessment (Update 17 December 2009)
  4. ECDC Forward Look risk assessment (Update 28 October 2010)