Obesity may be risk factor for severe H1N1 illness

Jul 10, 2009 (CIDRAP News) – In a series of 10 cases of severe illness with the pandemic H1N1 virus in Michigan, nine patients were obese, suggesting that very overweight people may be particularly vulnerable to life-threatening H1N1 infections, the Centers for Disease Control and Prevention (CDC) reported today.

Seven of the nine obese patients were considered extremely obese, and three of the nine patients died, according to the report, published as an MMWR Dispatch.

"Clinicians should be aware of the potential for severe complications of novel influenza (H1N1) virus infection, particularly in extremely obese patients," the CDC report says.

The report is not the first time that obesity has been cited as an unexpectedly common finding in patients with severe H1N1 illness, but good data on the topic are scarce, said Dr. Anthony Fiore, a medical epidemiologist in the CDC Influenza Division and a coauthor of the report.

"It's been noted in some of the data that's been received by the WHO [World Health Organization]," Fiore told CIDRAP News. "It's been a phenomenon that other people have noticed, but I don't think in a quantitative sort of way."

Series profile
The 10 patients described in the report were referred to the surgical intensive care unit (SICU) at the University of Michigan Health System between May 26 and Jun 18. The unit specializes in treating patients with acute respiratory distress syndrome (ARDS).

The patients ranged from 21 to 53 years old, with a median age of 46, and all but one were men. Nine were classified as obese on the basis of a body mass index over 30, and seven of them had an index of 40 or over, classified as extremely obese. Two of the obese patients—one of them a smoker—had asthma, and a third had chronic lung disease.

All 10 patients required advanced mechanical ventilation, and two were treated with extracorporeal membrane oxygenation (ECMO). Six of them also needed treatment for acute renal failure. Nine had multiorgan dysfunction syndrome when admitted to the SICU, and nine suffered from septic shock. Five patients were found to have pulmonary emboli at the time of admission or later.

All the patients received antiviral treatment, but the authors estimated that it was started a median of 8 days after symptom onset—well beyond the recommended 2-day window for greatest effectiveness.

As of Jul 8, two patients were still receiving ECMO or mechanical ventilation, five had been transferred back to the referring hospitals in stable condition, and three had died, the report says.

No evidence of bacterial infection was found in any of the patients, all of whom received antibiotic treatment before and after they were admitted to the SICU. But the authors say they couldn't exclude the possibility of undetected bacterial infection

The report says the overall findings suggest that the patients' lung damage was caused by primary viral pneumonia, but their own intense immune responses might have contributed as well.

Obesity prevalence called striking
"The high prevalence of obesity in this case series is striking," the report states.

It says it is unknown whether obesity is an independent risk factor for severe complications of H1N1 infection. Obesity has not been tagged as a risk factor for severe seasonal flu. However, one study showed that mice with diet-induced obesity were more likely to die of seasonal flu infections than lean mice were, the report states.

Also, it notes that extremely obese flu patients, compared with normal weight patients, have a higher prevalence of other health conditions that increase the risk of flu complications, including heart, lung, liver, and metabolic diseases.

"It might actually be an issue that this [obesity] is a marker of other underlying chronic illnesses," rather than an independent risk factor for H1N1 complications, said Fiore.

He said it's possible that obesity also confers a risk for severe complications in seasonal flu, but it just hasn't been identified before.

Fiore said the CDC is studying the issue further. "We're certainly going to look at the information in our hosptalization data from a national perspective. It's challenging because it's not the usual sort of medical risk factor that people check off. It doesn't show up in the usual codes like asthma or congestive heart failure."

"There's a large prevalence of obesity in the country right now, and of severe obesity, so we'd expect some flu patients to have those conditions, but we don't have a national figure," he commented. On the basis of telephone surveys, the CDC reported this week that 26.1% of Americans were obese in 2008, up from 25.6% in 2007.

CDC officials have previously mentioned obesity as a finding in some patients with severe H1N1 infections. In a May 19 press conference, Dr. Anne Schuchat commented, "We were surprised by the frequency of obesity among the severe cases that we′ve been tracking. I do think it′s an important result. The question of whether people with obesity need to be treated differently in terms of antiviral treatment or seasonal flu vaccinations is one we′re looking into."

Today's report notes that the 10 patients received oseltamivir treatment in higher doses and over a longer period than normal. It says that until more is known, increased oseltamivir regimens can be considered for severely ill H1N1 patients.

CDC. Intensive-care patients with severe novel influenza A (H1N1) virus infection—Michigan, June 2009. MMWR Dispatch 2009 Jul 10;58:1-4 [Full text]

See also:

Transcript of May 19 CDC press briefing

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