Obesity and H1N1

The Washington Post just ran a story with the headline "Survey finds link between obesity and flu severity."  Now, I'm usually a big fan of The Post, but that turns out to be completely inaccurate.  

The study in question, entitled "Hospitalized Patients with Novel Influenza A (H1N1) Virus Infection --- California, April--May, 2009," was published online as an early release by the CDC's Morbidity and Mortality Weekly Report (MMWR), and there is nothing in that study to indicate that obesity is a risk factor for severe disease with H1N1.  Nothing at all.  

The authors don't interpret their findings that way in the paper, and the simplest of math shows that such a conclusion makes no sense on the basis of this data.

As the wonderful XKCD t-shirt my wife got for me says, "Stand back, I'm going to try SCIENCE!"

The prevalence of obesity among the 30 hospitalized cases in California was 4/30 = 13.33%

The prevalence of obesity in California (in 2001) was about 20.9%

13.33% is less than 20.9%

Now, you shouldn't take that to mean that obesity is protective against the H1N1 virus.  (The numbers are just too small for statistical stability.)  But there's absolutely no reason on the basis of this data to conclude that obesity "obesity may raise risk for flu complications," as the Post's running title for this piece reads.  Which is why the authors make no such assertion.

Does that mean the same thing as "there's absolutely no reason at all to conclude that obesity is a risk factor for severe disease with H1N1?"  No, it does not.  Thirty cases are too few to draw definitive conclusions.  And, in fact, the article goes on to say:

"We were surprised by the frequency of obesity among the severe cases that we've been tracking," said Anne Schuchat, one of the CDC epidemiologists managing the outbreak. She said scientists are "looking into" the possibility that obese people should be at the head of the line along with other high-risk groups if a swine flu vaccine becomes available.

And it is entirely possible that there's something to that, though the article gives us no direct evidence to support it.

But if any such evidence exists, it certainly isn't to be found anywhere in the MMWR paper.  And the paper's authors make no claim that it is, so don't blame them for the inaccuracy.