How deadly is bird flu?

The Centers for Disease Control and Prevention, and the World Health Organization, say that H5N1 bird flu kills some 60% of the human beings it manages to infect. Basically, it hasn't infected many people—because it can't be spread from person to person—but most of the people it does infect die.

But this might not be the full story.

After I posted a summary of the current controversies surrounding H5N1 research, I got an interesting email from Vincent Racaniello, a professor of microbiology at Columbia University Medical Center. Racaniello points out that the 60% death rate statistics are based on people who show up at hospitals with serious symptoms of infection. So far, there've only been about 600 cases. And, yes, about 60% of them have died.

However, they don't necessarily represent everybody who has contracted H5N1.

A death rate is only as good as statistics on the rate of infection. If you've got an inaccurate count of the number of people infected, your death rate is going to be wrong. And there's some evidence that might be the case with H5N1.

In a recent study of rural Thai villagers, sera from 800 individuals were collected and analyzed for antibodies against several avian influenza viruses, including H5N1, by hemagglutination-inhibition and neutralization assays. The results indicate that 73 participants (9.1%) had antibody titers against one of two different H5N1 strains. The authors conclude that ‘people in rural central Thailand may have experienced subclinical avian influenza virus infections’. A subclinical infection is one without apparent signs of illness.

If 9% of the rural Asian population has been subclinically infected with avian H5N1 influenza virus strains, it would dramatically change our view of the pathogenicity of the virus. Extensive serological studies must be done to determine the extent of human infection with avian H5N1 influenza viruses. Until we know how many individuals are infected with avian influenza H5N1, we must refrain from making dire conclusions about the pathogenicity of the virus.


  1. “Subclinical” for this study doesn’t even necessarily mean that the disease was symptomless – it just means that the symptoms weren’t severe enough for the patient to see a medical professional. If I (a middle class Briton (ie even with free healthcare)) got symptomatic, stay-in-bed-moaning-for-a-few-days flu, the closest I’d get to medical care would probably be texting a friend to ask them to get me some Lucozade – especially if I wasn’t aware it was likely to be an easily transmissable, pandemic strain. I suppose seeing a medical professional with unpleasant but non-lifethreatening flu is even less likely for poor, rural Thais.

  2. I just moved to Hong Kong from the US about a month ago, and observed how the people and the media are deathly afraid of a bird flu pandemic. Last week, I became very ill with a fever, and after sustaining a constant 38 degree temp for about 5 days decided to visit a health clinic. The doctors there confirmed that I had a fever (really? Thanks!) and gave me some Tylenol and sent me on my way. But at no point was a sample taken to determine which strain of a virus I had developed. I’m still quite sick and I actually came to the same conclusion as your article as I left the clinic; no one knows how many people have bird flu in Hong Kong, but a lot of us are wearing masks!

  3. I always thought that the mortality is reversely proportional to infectivity.  The virus is a dangerous killer when living deep in the respiratory tract, in the lungs. That also means that it has longer way out of there – less virus particles get sneezed out, infecting others. However, if the virus infect only upper respiratory tract, being less dangerous, the infectivity goes up.  So you have either a fast spreading nuisance or a slow killer, not both.

  4. While I think the science reasoning behind this skeptical approach is sound, the policy position is irresponsible. 

    We don’t spend enough energy on counter-flu research and vaccines, or treat illness as a whole with proper caution.  On one side is excessive fear based on a misunderstanding of the science, but on the other is anti-science suppression of necessary work that needs to be done for health and safety. We need more anti-virals and flu research, and stories like this that promote “D’awww, it’s not all that bad…” push the public into a false sense of security. 

    I believe that the CDC wisely operates on the precautionary principle, and since they are not calling for us to tape up our faces and homes with plastic wrap, only to handle this virus as a potential world changer when studying it, I am inclined to side with them…. even understanding the preliminary science may show lower risk.

  5. One data point here but I was one of the people who contracted the Avian H1N1.  I was travelling in SE asia (Malaysia.) and have a habit of visiting any interesting food markets and think I must have contracted it there.  I did not come down with anything until I landed in Chicago on my way home and had a high fever and the usual set of bad flu like symptoms.  I’m still not sure why customs didn’t stop me but they didn’t and I went on home.  

    Since I had been traveling in a possible Malaria/Dengue fever zone I went to the doctor.  Who recommended the usual sort of thing (tylenol/advil rotation, rest, liquids ect.) By the time the positive test came back I was feeling better.  At no time did I feel like I was going to die but a few times I wish I had.  Honestly, it was just a really bad flu.  No one else in my family contracted it and all was fine. 

  6. But this is nothing unique to bird flu – it’s the case for most infectious diseases that doesn’t have utterly clear symptoms. So the deeper question becomes: Is H5N1 *more* underdiagnosed than whatever disease we are comparing the death rate with?

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