36,000 people die from flu annually in the U.S? Probably not.

There's a statistic I've been hearing a lot lately: according to the CDC, 36,000 Americas die from the flu every year. Mostly I've been hearing this from (well-intentioned) people pushing flu vaccination.

(Please note that this figure is about the regular seasonal flu, not the H1N1 strain, and -- except for one note below -- I'm not commenting on H1N1 here.)

Now that's a heck of a figure. 36,000 a year? If that's right, then every two years more Americans die from flu than were killed in Vietnam -- there are 58,195 names on the Vietnam Veterans Memorial Wall.

2,993 people were killed in the 9/11 attacks: if the 36,000 figure is right, every year the flu is a dozen 9/11s.

Now, that seems odd. I'm too young to know anyone who died in Vietnam, but I knew two people who died in the 9/11 attacks. Of course, that's a non-random distribution -- I live on the East Coast -- but I don't think I know anyone who's died from the flu. If in my adult life I've lived through 240 9/11's worth of flu deaths, it seems like I ought to know somebody affected. These numbers don't seem to make sense, and my skeptic bone is starting to itch.

So where does this 36,000 figure come from? Do they actually test people who die from flu-like symptoms for the influenza virus and count them? Well, no. According to their own website, "CDC does not know exactly how many people die from seasonal flu each year."

And they also admit that the 36,000 figure is not deaths caused by flu, but "flu-related" deaths:

Seasonal flu-related deaths are deaths that occur in people for whom seasonal influenza infection was likely a contributor to the cause of death, but not necessarily the primary cause of death.

Keep this in mind as you hear about deaths supposedly from the H1N1 pandemic: most of these will never be verified by any hard evidence that H1N1 infection was the primary cause of death. Instead, the more people think the H1N1 is a killer, the more they will attribute ambiguous deaths to H1N1. It's the same principle that makes the Law of Fives work.

So the CDC's 36,000 figure is not based on actually counting deaths caused by flu, but based on the use of a statistical model to guess at the number of "flu-related" deaths, because otherwise they'd get (in their opinion) too low of a count:

Seasonal influenza may lead to death from other causes, such as from congestive heart failure or chronic obstructive pulmonary disease, as was observed during the 1957-1958 pandemic. It has been recognized for many years that influenza is infrequently listed on death certificates and testing for seasonal influenza infections is usually not done, particularly among the elderly who are at greatest risk of seasonal influenza complications and death. Some deaths – particularly in the elderly – are associated with secondary complications of seasonal influenza (including bacterial pneumonias). Influenza virus infection may not be identified in many instances because influenza virus is only detectable for a short period of time and many people don’t seek medical care until after the first few days of acute illness. For these reasons, statistical modeling strategies have been used to estimate seasonal flu-related deaths for many decades, both in the United States and the United Kingdom. Only counting deaths where influenza was included on a death certificate would be a gross underestimation of seasonal influenza’s true impact.

Is their statistical model worth a hill of beans? According to this analysis published in the BMJ, quite likely not:

The CDC website states what has become commonly accepted and widely reported in the lay and scientific press: annually "about 36 000 [Americans] die from flu" (www.cdc.gov/flu/about/disease.htm) and "influenza/pneumonia" is the seventh leading cause of death in the United States (www.cdc.gov/nchs/fastats/lcod.htm). But why are flu and pneumonia bundled together? Is the relationship so strong or unique to warrant characterising them as a single cause of death?

David Rosenthal, director of Harvard University Health Services, said, "People don't necessarily die, per se, of the [flu] virus—the viraemia. What they die of is a secondary pneumonia. So many of these pneumonias are not viral pneumonias but secondary [pneumonias]." But Dr Rosenthal agreed that the flu/pneumonia relationship was not unique. For instance, a recent study (JAMA 2004;292: 1955-60[Abstract/Free Full Text]) found that stomach acid suppressing drugs are associated with a higher risk of community acquired pneumonia, but such drugs and pneumonia are not compiled as a single statistic.

CDC states that the historic 1968-9 "Hong Kong flu" pandemic killed 34 000 Americans. At the same time, CDC claims 36 000 Americans annually die from flu. What is going on?

Meanwhile, according to the CDC's National Center for Health Statistics (NCHS), "influenza and pneumonia" took 62 034 lives in 2001—61 777 of which were attributed to pneumonia and 257 to flu, and in only 18 cases was flu virus positively identified. Between 1979 and 2002, NCHS data show an average 1348 flu deaths per year (range 257 to 3006). [Emphasis added. -tms]

The NCHS data would be compatible with CDC mortality estimates if about half of the deaths classed by the NCHS as pneumonia were actually flu initiated secondary pneumonias. But the NCHS criteria indicate otherwise: "Cause-of-death statistics are based solely on the underlying cause of death... defined by WHO as `the disease or injury which initiated the train of events leading directly to death.'"

Confusing "flu caused" deaths with "flu and pneumonia related" deaths would be merely sloppy science, if it weren't for the public health implications. This figure of 36,000 flu deaths a year is being used to drive the seasonal flu vaccination push -- which seems to be at best mostly a waste of resources that could be put more effective, more life-saving use elsewhere.

If in fact the actual figure is more like 1348, then more than twice as many people drown each year, and more people die from falling on the stairs than die from flu.

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