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Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
  • The English language content on this website is being archived for historic and reference purposes only.
  • For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

Questions & Answers

2009 H1N1 Flu In The News

March 16, 2010 10:30 AM ET

Morbid Obesity as a Risk Factor for Hospitalization and Death due to 2009 H1N1

This document provides updated information on obesity and morbid obesity as risk factors for serious 2009 H1N1-related complications based on findings from recent studies.

How are obesity and morbid obesity defined?

Body mass index (BMI) is a measure of body fat based on height and weight. Obesity is defined as a BMI of greater than or equal to 30 kilos per meter squared and morbid obesity is defined as a BMI greater than or equal to 40 kilos per meter squared. Among Americans 20 years and older, 28% are obese and 5% are morbidly obese.

Have obesity or morbid obesity been considered risk factors for serious flu-related complications in the past?

The Advisory Committee on Immunization Practices considers that certain chronic health conditions place persons at higher risk of serious flu related complications, including asthma, diabetes and heart disease, and emphasizes the importance of vaccination in people with these health conditions. In the past, neither obesity or morbid obesity have been considered independent risk factors that would place people at higher risk for serious flu-related complications.

What has been learned from the 2009 H1N1 pandemic about obesity and risk of serious influenza disease death?

During the 2009 H1N1 pandemic, early reports from the United States and abroad suggested that obesity was more frequent among persons hospitalized with 2009 H1N1 disease or who died following 2009 H1N1 infection.

Since that time, a number of studies have suggested that many 2009 H1N1patients tend to be morbidly obese. The study “Morbid Obesity as a Risk Factor for hospitalization and Death due to 2009 Pandemic Influenza A (H1N1) Disease,” published in PLoS ONE, sought to determine whether or not obesity or morbid obesity were in fact independent risk factors for serious 2009 H1N1-related complications, including death.  This study found that morbidly obesity persons have a higher risk of hospitalization for 2009 H1N1 infection compared to persons with normal weight. Data from this study also suggest that the risk of death following H1N1 infection may be higher for morbidly obese individuals.

What has been learned from the 2009 H1N1 pandemic about morbid obesity and risk of death?

People who are obese or morbidly obese may be at a higher risk of dying from 2009 H1N1, even without any other previously recognized high risk conditions.

Should obesity be considered a high risk condition?

Scientific findings to date, including the PLoS ONE study, support the conclusion that people who are morbidly obese are at increased risk from complications of 2009 H1N1; these individuals should get vaccinated against it and should be treated early with antivirals if they do become sick. It is unclear based on this study, however, if a casual relationship exists between obesity and severe complications from 2009 H1N1 independent of other factors.

What should morbidly obese people do based on these findings?

Getting a flu vaccine is the most important action people can take to prevent influenza and its complications.  Because people who are morbidly obese are at a higher risk of serious 2009 H1N1-related complications, it’s especially important that they get vaccinated against 2009 H1N1.

With the ACIP recommendation on February 24, 2010 to adopt universal influenza vaccination for the 2010-2011 season, everyone 6 months and older is now recommended to get an annual flu vaccination.  Next year’s seasonal influenza vaccine will contain the 2009 H1N1 virus, in addition to two other viruses that research indicates are most likely to circulate during the upcoming season.

Losing weight by eating healthier and getting moderate physical activity several times a week can also improve one’s overall health and may lessen chances for developing severe outcomes from the flu.

What should clinicians do in terms of treating morbidly obese patients based on these findings?

Clinicians should evaluate morbidly obese patients with possible influenza illness for the presence of chronic medical conditions.  If these conditions are present, or if persons with morbid obesity show signs of lower respiratory tract infection or other signs of severe illness, these patients should receive empiric antiviral treatment as early as possible.  Earlier treatment of people with influenza who have a high risk condition such as morbid obesity can decrease the risk of severe illness and death.

Should morbid obesity be considered a high risk condition?

Yes. Morbid obesity, with or without the presence of other underlying high risk health conditions, is associated with a higher risk of 2009 H1N1-related hospitalization and, possibly death.

People who are morbidly obese often suffer from other medical conditions and it is possible that some people who are morbidly obese have unrecognized chronic medical conditions.

What other studies exist to support the conclusion that morbid obesity is a high risk factor?

Other recently published data discusses obesity as a high risk factor severe for 2009 H1N1-related complications:

  1. Louie JK, Acosta M, Winter K, et al. Factors associated with death or hospitalization due to pandemic 2009 influenza A(H1N1) infection in California. JAMA 2009;302:1896-902.
  2. Webb SA, Pettila V, Seppelt I, et al. Critical care services and 2009 H1N1 influenza in Australia and New Zealand. N Engl J Med 2009;361:1925-34.
  3. Miller RR, 3rd, Markewitz BA, Rolfs RT, et al. Clinical findings and demographic factors associated with intensive care unit admission in Utah due to 2009 novel influenza A (H1N1) infection. Chest 2009;[epub ahead of print].
  4. Kumar A, Zarychanski R, Pinto R, et al. Critically ill patients with 2009 influenza A(H1N1) infection in Canada. JAMA 2009;302:1872-9.

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