Study confirms extreme obesity as novel H1N1 risk factor

Jan 5, 2011 (CIDRAP News) – Obesity as a risk factor for severe illness and death was one of the clinical surprises that emerged early in the 2009 H1N1 pandemic, and now California researchers have shown that extreme obesity is an independent risk factor for death from the disease.

They found that extremely obese patients, those with a body mass index (BMI) of 40 or more, were three times more likely to die from their 2009 H1N1 infections compared with those whose BMI fell in the normal range, according to the report, published in an early online edition of Clinical Infectious Diseases.

Obesity as a risk factor with severe flu infections hasn't been linked to seasonal influenza, and when early signs showed that it may be a risk factor for 2009 H1N1 complications, health experts weren't sure if other underlying health conditions played a role or if something about being obese, such as breathing compromise, made it an independent risk factor.

The researchers from the California Pandemic (H1N1) Working Group based their findings on enhanced surveillance for hospitalized and fatal 2009 H1N1 cases that the state initiated soon after the first two cases were identified in southern California in April 2009. Their report today covers data collected between Apr 20 and Aug 11, 2009.

In November 2009 the same group fleshed out some of the first clinical features of the new virus in a Journal of the American Medical Association (JAMA) study, showing the virus was hitting young people the hardest and that obesity may be a risk factor. However, in their latest report they took a more detailed demographic and clinical look at California's sickest 2009 H1N1 patients during the first 16 weeks of the pandemic and conducted a multivariate analysis to see if obesity stood out as an independent risk factor.

The latest study includes 534 adult patients who had 2009 H1N1 infections and for whom BMI data were available. Half (51%) of the patients older than 20 who were hospitalized were obese. Of the 92 patients who died, 56 (61%) had BMIs of 30 or more and 28 (30%) had BMIs of 40 or more. The prevalence of BMIs of 40 or more in the patients in the case series was 2.2 times and 1.5 times greater than estimated for the California and US populations, respectively.

Multivariate analysis associated the following factors with death: BMI of 40 or more (odds ratio [OR], 2.8; 95% confidence interval [CI], 1.4-5.9); age 50 or older (OR 2.1, CI 1.2-3.7); certain immunosuppressive conditions such as congenital immunodeficiency, asplenia, and adrenal disorders (OR 3.9, CI 1.6-9.5); and asthma (OR 0.5, CI 0.3-0.9).

Analysis showed that those with BMIs of 45 or more had an even greater 4.2-fold increased odds of death from 2009 H1N1 influenza.

The authors also point out that the data do not suggest that moderately obese patients without other risk factors and hospitalized with the 2009 H1N1 virus have an increased risk of dying compared with those who are not obese.

More than three fourths of obese patients in the study group received antiviral medications, but while nonobese and obese patients did not vary in onset of symptoms to hospitalization, obese patients were more likely to be treated later, the investigators found. They added that more data are needed on whether higher dose and longer duration treatment with neuraminidase-inhibiting antiviral drugs like oseltamivir (Tamiflu) and zanamivir (Relenza) might improve clinical outcomes in obese patients who have seasonal or 2009 H1N1 flu.

The authors cautioned that their data could be limited by nonstandardized medical records, passive reporting by clinicians, and underreporting of influenza, but they said none of the limitations would introduce selection bias. They said height and weight data were somewhat more available for fatal versus non fatal cases, 95% versus 85%. They added that there were differences in patients with and without BMI data, but none of the factors linked to the missing BMI data were associated with death in the multivariate analysis.

Study author Janice Louie, MD, MPH, with the California Department of Public Health, said in a press release from the Infectious Diseases Society of America (IDSA), said that extremely obese people, those with BMIs of 40 or more, should receive annual flu immunizations.

"They should also see their health provider early if symptoms of influenza develop, so that they can get diagnosed and treated as quickly as possible. This is especially important if the influenza virus is known to be circulating in the community and causing illness," she said.

The study group suggested that BMI measurements can be used to identify adults at high risk for severe disease and that BMI categories could be added to prioritization groups for antiviral use and influenza vaccine when supplies are scarce.

Louie said more research is needed to clarify why extremely obese people are more likely to die from 2009 H1N1 infections.

Louie JK, Acosta M, Samuel MC, et al. A novel risk factor for a novel virus: obesity and 2009 pandemic influenza A (H1N1). Clin Infect Dis 2011 Feb 1;52(3):300-11 [Abstract]

See also:

Jan 5 IDSA press release

Nov 3, 2009, CIDRAP News story "Obesity risk stands out in study of California's sickest H1N1 patients"

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