Wednesday, February 28, 2018

Study: Obesity & Influenza-Like-Illness (ILI) Severity


Early reports during the 2009 H1N1 pandemic (see H1N1 Morbidity And Previously Existing Conditions) cited unusual numbers of obese influenza patients - a previously unrecognized risk group - populating intensive care facilities around the world. 
Over the next couple of months obesity, and particularly morbid obesity, was frequently mentioned as a possible risk factor, along with asthma, diabetes, and immune disorders.   
That is, until an ACIP meeting held at the end of July, where evidence was presented that showed that the incidence of hospitalizations among those listed as obese by their BMI was practically the same as their prevalence in society.
Roughly 34% of Americans are obese, and roughly 38% of those hospitalized met that criteria.  While 6% are morbidly obese (BMI > 40), they only made up 7% of the hospitalized cases.
Since then, the debate has raged on, with some studies showing an elevated risk of severe influenza, hospitalization, and death among those who are morbidly obese, while other studies failing to find such evidence. 
Alas, science isn’t always neat, tidy, consistent or clear (see When Studies Collide (Revisited)).
In September of 2009, in Study: Half Of ICU H1N1 Patients Without Underlying Conditions, it became apparent that while pre-existing risk factors were important, they were not the sole reason behind flu victims ending up in intensive care.

In November of that year, Eurosurveillance Journal  published a Study: H1N1 Hospitalization Profiles, that similarly found :
  • The most common risk factor in admission to intensive care was chronic respiratory disease followed by chronic neurological disease, asthma and severe obesity.
  • 51% of hospitalized cases and 42% of ICU cases were not in a recognized risk group.
Over time, while studies were far from consistent, the tide began to turn, with morbid obesity increasingly cited as a risk factor for severe influenza.

In early 2011 (see Extreme Obesity: A Novel Risk Factor For A Novel Flu) the IDSA’s journal Clinical Infectious Diseases carried a study called  A Novel Risk Factor for a Novel Virus: Obesity and 2009 Pandemic Influenza A (H1N1), that found:
Extreme obesity associated with higher risk of death for 2009 H1N1 patients
[EMBARGOED FOR JAN. 5, 2011] For those infected with the 2009 pandemic influenza A (H1N1) virus, extreme obesity was a powerful risk factor for death, according to an analysis of a public health surveillance database.
In a study to be published in the February 1, 2011, issue of Clinical Infectious Diseases, researchers associated extreme obesity with a nearly three-fold increased odds of death from 2009 H1N1 influenza. Half of Californians greater than 20 years of age hospitalized with 2009 H1N1 were obese. 
 For the last few years the CDC has cited `people with extreme obesity (i.e., body-mass index is equal to or greater than 40)' on their list of High Risk groups for severe influenza.
Today, via Nature, we have a pre-release abstract from the International Journal of Obesity, whose findings I suspect are going to be viewed as controversial. 
Over a four year (2010-2014) study of ER visits for influenza-like-illness (ILI), they found that obesity was not linked to a greater risk of hospitalization - in fact - they found the opposite was true. 
Epidemiology and population health
Is weight associated with severity of acute respiratory illness?
Elizabeth E. Halvorson,Timothy R. Peters, Joseph A. Skelton, Cynthia Suerken, Beverly M. Snively & Katherine A. Poehling

International Journal of Obesity (2018)
Published online: 27 February 2018



Obesity was an independent risk factor for severe disease in hospitalized adults during the 2009 pandemic H1N1 influenza season. Few studies have investigated the association between weight and severity of acute respiratory illnesses in children or in adults seeking care in the emergency department (ED) during other winter respiratory seasons.

We prospectively and systematically enrolled patients ≥2 years of age who presented to the ED or inpatient setting in a single geographic region with fever/acute respiratory illness over four consecutive winter respiratory seasons (2010–2014).
We collected demography, height and weight, and high risk co-morbid conditions. Multivariable logistic regression was used for prediction of hospital admission (primary outcome), length of stay and supplemental oxygen requirement among those hospitalized, and antibiotic prescription (secondary outcomes).

We enrolled 3560 patients (N = 749 children, 2811 adults), 1405 (39%) with normal weight, 860 (24%) with overweight, and 1295 (36%) with obesity. Following multivariable logistic regression, very young or very old age (p < 0.001) and high-risk conditions (p < 0.001) predicted hospitalization.
Risk of hospitalization was decreased for adults with overweight [aOR 0.8 (95% CI 0.6–1.0)], class 1 obesity [aOR 0.7 (95% CI 0.5–1.0)], and class 2 obesity [aOR 0.6 (95% CI 0.4–0.8)] compared to normal-weight. Class 3 obesity was associated with supplemental oxygen requirement in adults [aOR 1.6 (95% CI 1.1–2.5)]. No association was seen in children.

Overweight and obesity were not associated with increased risk of hospitalization during winter respiratory seasons in children or adults.
While the results are interesting, this is but one study done in a single location, and was conducted after the 2009 H1N1 pandemic.  It uses `fever/acute respiratory illness' as a benchmark, and not lab-confirmed influenza. 
Being personally somewhat `gravitationally challenged', I'd very much like this to be true.
But until these results can be fully examined, and replicated by others, the best we can say right now is  . . .  of all of the studies on obesity as a risk factor for severe influenza . . .  this is without a doubt, the most recent.     

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