Obesity Still Dramatically Decreases Risk of Severe H1N1 Flu?

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“[1]. 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.

[1] Justin T Denholm et al, eMJA Rapid Online Publication – 16 November 2009

Categories: medicine, Science, skepticism

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6 replies

  1. The largest series I’m aware of is the recent JAMA study, Factors Associated With Death or Hospitalization Due to Pandemic 2009 Influenza A(H1N1) Infection in California (JAMA. 2009;302(17):1896-1902. ). (The eMJA paper came about about simultaneously, so they can be forgiven for claiming theirs is the largest study.) Here 48% (of hospitalized or fatal cases, where data were recorded) were obese. The rate was slightly higher in fatal cases (66%, of 110 cases total) than non-fatal (52%, of 212 cases; these are cases over 18 years, and the non-fatal rate would be lower if we included younger people). The fatal cases, especially, were disproportionately in the highest BMI cases — 50% of fatal cases had a BMI over 40, if I’m reading their Table 2 right.
    I don’t know what the relevant population rates of obesity are, so we don’t know relative risk. In the US generally, I believe obesity is in the 30% range. The authors say “Of adults with BMI data available, more than half were obese and one-quarter were morbidly obese. As a point of reference, the percentage of adults who are morbidly obese in the United States is 4.8%”.
    (An important concern is that this may be distorted. It looks as if data weren’t recorded for obesity on the majority of patients. I would worry about a recording bias, with information on obese patients being recorded more readily than for non-obese. Still, even if not one of the non-recorded patients had BMI over 40, the case rate is higher than the 4.8% background.)
    In smaller studies, there seems to be a similar picture. In a Michigan survey (June ’09) 9 of the 10 patients with swine-origin H1N1 hospitalized with ARDS were obese (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5827a4.htm); in a European survey, 8 of 13 fatal cases were obese (http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19309).
    The numbers are still quite small, and they’re not all consistent, but from what I see here, I wouldn’t dismiss obesity as a risk factor.

  2. … Also, I see a survey in Australia (Progression and impact of the first winter wave of the 2009 pandemic H1N1 influenza in New South Wales, Australia; Eurosurveillance, Volume 14, Issue 42, 22 October 2009) where the relative risk of obesity and “morbid obesity” (BMI > 40) is worked out. Just as you note, the relative risk of “obesity” for death is less than 1 (0.6), but probably 1 is easily in the 95% confidence interval. But the RR for obesity of ICU admission is up (1.7) and for morbid obesity is 4.4; death RR for the latter is 2.4.
    These data aren’t entirely consistent with the eMJA data, but I don’t have time to try to resolve the differences. One point that’s raised by the JAMA study I quoted above is whether BMI is specifically recorded in these cases. The eMJA study only notes 8 patients that were obese, but unlike the JAMA study they don’t explicitly give a denominator — that is, they don’t specifically say that the remaining 104 patients had a BMI recorded at all. Could the BMI simply not be available for some of these remaining patients? I don’t know, I’m asking.

  3. iayork: There is something drastically hinky about these data, viewed together, isn’t there? Denominators? Prejudice? Something else? And it appears to be something that the authors don’t seem to be (publicly?) acknowledging or examining.
    I don’t discard obesity as a risk factor out of hand – but I saw the CDC and other ‘experts’ at the beginning deciding that obesity was obviously a major risk factor based on early data that appeared to show the complete opposite – and everything that’s happened since then with this issue appears to lack rigor as well. The pregnancy-as-risk-factor issue seems very clear. This, not so much. I’m a bit less interested, in this space, on _whether_ it is an independent risk factor, than on _why_ the data are being ignored/overlooked/massaged in the ways that they are, first by scientists and then by PR machines, government, and media.

  4. For what it’s worth, neither the CDC MMWR article from May, nor the eMJA paper that just came out, say that obesity is a factor. The CDC report includes obesity last in a list of underlying medical conditions, and never says that it’s a risk factor per se. The eMJA paper only mentions obesity when they define it, show the rate in a table, and don’t mention it in their discussion at all.
    So I’m not sure you can blame the scientists here. The overwrought press coverage, as far as I can see, entirely arose out of a comment by Schuchat in a press briefing. (Schuchat would not be one of the authors of the article.). Her comment certainly was misleading, but it’s not quoting the scientists who did the work; it seems to come out of nowhere. Not excusing her here, but I would bet her comment was in response to a specific question from the press, not something she raised herself, and she seems to have only been referring to “severe cases” (not all the cases in that MMWR report), which at that time would have been a tiny subset of a tiny subset of cases.
    And she corrected herself, at least partially, later on. If you look at a subsequent press briefing (in July, http://www.cdc.gov/media/transcripts/2009/t090729b.htm) she specifically says that the difference is not there, especially accounting for other underlying conditions, and “They [the obese] would not be a targeted group.” That didn’t get any press, as far as I can find.
    So, not surprisingly, the press has done a poor job of covering this, jumping on the comment from Schuchat without checking the figures. The experts, at least those who are actually doing the work, aren’t making the connection except in those studies that actually do see a disproportionate number (the JAMA study and others).

  5. iayork: please don’t use “the obese” here; this language is not acceptable.
    That press conference didn’t penetrate the media significantly – I saw no corrections anywhere at all. Certainly the government/medical authorities here in Aus failed to get the memo – “moderate to severe obesity” is listed as a risk group prioritised for immunisation.
    Schuchat’s original statements weren’t a single throwaway remark – as I recall, she went into some detail about her perceptions of obesity and risk, including regurgitating rubbish about there being an “obesity epidemic”.

  6. Schuchat’s original statements weren’t a single throwaway remark
    That’s true, but in fact the context of her explanation was exactly the point you are making — that the frequency of obesity in SOIV patients wasn’t necessarily higher than that in the population (“So it’s hard for us to say at this point to say whether the number of patients with reported obesity is significantly higher than we would expect”). In other words, I’d say that “her perceptions of obesity and risk” were pretty much what you’re saying.
    That was the May press briefing, the one that led to the press rampage. Now, reading the press conference transcript (http://www.cdc.gov/media/transcripts/2009/t090519.htm), the point she tried to make didn’t come across very well, because she started off sounding as if she agreed with the obesity issue and didn’t make the qualifications until several questions later. She clearly recognized that she hadn’t been clear, because she tried to clarify the point in each of the subsequent briefings (in June and in July). But I’m not seeing this as the experts making assumptions — quite the opposite, in fact. The expert was carefully not making the assumption, but the press didn’t pick up on the qualifiers that she explicitly presented. She could have presented this better, but I’m inclined to put it down to imperfect communication, not jumping to conclusions.

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