I wrote about this before, with very preliminary data, very small numbers – the June figures from the CDC on obesity as a risk factor for swine flu, which was showing that the prevalence of obesity in the ‘severe’ group was actually a lot less than the prevalence of obesity in the general population. The CDC misinterpreted the numbers, in a giant clanger that no mainstream media called them out on, to say that obesity was a ‘risk factor’ for severe H1N1 influenza.
“What’s happened since then?” you ask. “Surely we have more numbers, local numbers?”
Local numbers for the first two months of the epidemic hit the Medical Journal of Australia this week, in “Hospitalised adult patients with pandemic (H1N1) 2009 influenza in Melbourne, Australia“. Let’s have a look at Table 1, which lists the prevalence of various ‘risk factors’.
Demographic characteristics and comorbid conditions of the first 112 patients admitted to participating hospitals with pandemic (H1N1) 2009 influenza
Obesity 8 (7%)
Let’s have a look at the paper’s authors’ definition of ‘obesity’, just to double-check:
Obesity was defined as a body mass index of > 30 m/kg
And let’s have a look at the prevalence of obesity in the Australian population:
In the 2005 National Health Survey, 53.6% of Australians reported being overweight (above a 25 BMI), with 18% falling into the “obese” category (above a 30 BMI).
Note that this study was in Victoria, which has the lowest prevalence of obesity in Australia, at 17%.
That must be corrected for age, you say? OK. The median age of these 112 hospitalised adults with H1N1 flu was 42. Peeking at age-specific prevalence of obesity in Australia, the prevalence in the 35-44 group is 18.6 – almost exactly the national average for adults.
So. Around 18% of the Australian population is obese. Around 7% of people severely ill with H1N1 flu are obese.
Why is no-one looking closely at these data to see if possibly, just possibly, obese people are at lower risk for severe influenza? Couldn’t there be some useful biological/immune knowledge hidden in this recurring factoid? Why are we panicking globally about flu, but conveniently refusing to see some of the data?
There are others numbers from elsewhere, different data sets, showing an increase in risk of hospital admission where people are obese. (There’s a tricky confounder in there, in that once you start telling doctors obesity is a risk factor, fat people presenting with similar symptoms to thin people may be more likely to be admitted. There’s another confounder, more easily controlled for, which is that some underlying illnesses may be likely to make people more obese: good statisticians need to pull obesity out and see whether it’s an independent risk factor.) Meanwhile, also, pregnancy, asthma, and smoking continue to raise the risk of severe flu. But you’ll read about that in the papers.
 Justin T Denholm et al, eMJA Rapid Online Publication – 16 November 2009