My friend Lauredhel, at the Hoyden About Town blog, made an interesting point about risk factors for swine-origin influenza virus (SOIV), and the perception of those risk factors in the press.   The press has made a big deal of the putative link between obesity and risk of severe SOIV.  But, as she pointed out back in June, the data1 at that point showed no such link — in fact the percent of obese people with severe SOIV was if anything lower than the frequency of obesity in the general population.  However, the press picked up on this because of a reply to a question at the May CDC press briefing, and the headlines were all over this (actually non-existent) link.

Lauredhel made the same point when a recent paper in the Medical Journal of Australia reviewed cases of SOIV. 2  Once again, Lauredhel points out, in this series of patients, obesity was actually lower in severely affected people than in the general population:

Around 18% of the Australian population is obese. Around 7% of people severely ill with H1N1 flu are obese. 3

However, I don’t think the story is that simple.  I made a few comments there, which I’ll review here.  The summary of the exchange, I think (Lauredhel might disagree) is that some, but not all, studies have found a connection between obesity and risk of severe SOIV; the largest studies do show a connection, but when looking at the overall picture, it’s not a strong connection.  The press has, however, vastly overstated this link, focusing on it apparently because of one comment in a press briefing, but ignoring several attempts to clarify and downplay the observation.

My comments:

The largest series I’m aware of is the recent JAMA study. 4 (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%”.5

(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; in a European survey, 8 of 13 fatal cases were obese.

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.

Also, I see a survey in Australia6 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.

Lauredhel said in the comments:

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

I replied:

For what it’s worth, neither the CDC MMWR article from May, nor the eMJA paper that just came out,7 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 Anne 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) she specifically says that the difference is not there, especially accounting for other underlying conditions, and “They [obese people] 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).

Lauredhel said:

Schuchat’s original statements weren’t a single throwaway remark

I replied:

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, 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.


  1. Centers for Disease Control and Prevention (CDC) (2009). Hospitalized patients with novel influenza A (H1N1) virus infection – California, April-May, 2009. MMWR. Morbidity and mortality weekly report, 58 (19), 536-41 PMID: 19478723[]
  2. Justin T Denholm, Claire L Gordon, Paul D Johnson, Saliya S Hewagama, Rhonda L Stuart, Craig Aboltins, Cameron Jeremiah, James Knox, Garry P Lane, Adrian R Tramontana, Monica A Slavin, Thomas R Schulz, Michael Richards, Chris J Birch, & Allen C Cheng (2010). Hospitalised adult patients with pandemic (H1N1) 2009 influenza in Melbourne, Australia The Medical Journal of Australia, 192, 1-3[]
  3. Obesity Still Dramatically Decreases Risk of Severe H1N1 Flu?[]
  4. Louie JK, Acosta M, Winter K, Jean C, Gavali S, Schechter R, Vugia D, Harriman K, Matyas B, Glaser CA, Samuel MC, Rosenberg J, Talarico J, Hatch D, & California Pandemic (H1N1) Working Group (2009). Factors associated with death or hospitalization due to pandemic 2009 influenza A(H1N1) infection in California. JAMA : the journal of the American Medical Association, 302 (17), 1896-902 PMID: 19887665[]
  5. Louie JK, Acosta M, Winter K, Jean C, Gavali S, Schechter R, Vugia D, Harriman K, Matyas B, Glaser CA, Samuel MC, Rosenberg J, Talarico J, Hatch D, & California Pandemic (H1N1) Working Group (2009). Factors associated with death or hospitalization due to pandemic 2009 influenza A(H1N1) infection in California. JAMA : the journal of the American Medical Association, 302 (17), 1896-902 PMID: 19887665[]
  6. New South Wales public health network (2009). Progression and impact of the first winter wave of the 2009 pandemic H1N1 influenza in New South Wales, Australia. Euro surveillance : bulletin europeen sur les maladies transmissibles = European communicable disease bulletin, 14 (42) PMID: 19883546[]
  7. Justin T Denholm, Claire L Gordon, Paul D Johnson, Saliya S Hewagama, Rhonda L Stuart, Craig Aboltins, Cameron Jeremiah, James Knox, Garry P Lane, Adrian R Tramontana, Monica A Slavin, Thomas R Schulz, Michael Richards, Chris J Birch, & Allen C Cheng (2010). Hospitalised adult patients with pandemic (H1N1) 2009 influenza in Melbourne, Australia The Medical Journal of Australia, 192, 1-3[]