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October 14, 2005
Required Avian Influenza Reading
While I mentioned this new Sherry Cooper report on the economic impact of avian influenza at the tail end of another post recently, having just finished reading it I want to give the piece more prominence. It is very good — and absolutely harrowing.
Required reading.
On a related note, Cooper rightly worries about H5N1 in terms of its likelihood of inducing a cytokine storm analogous to that caused by the devastating Spanish flu of 1918 that killed so many millions worldwide. After all, the 1918 pandemic flu was also a bird flu, and more than half of its victims were young and healthy people — a fact that generally comes as a surprise to complacent 20–40 year-olds today who think that the elderly and the infirm are most at risk.
But that said, one of the more interesting questions is why hasn’t the current H5N1 virus reassorted with a human influenza virus, producing a form both lethal and able to be efficiently transmitted human-to-human? After all, it has been around for at least nine years, and untold numbers of people have been exposed to the virus, and yet H5N1 has not yet found a way to be efficiently transmitted. As Klaus Stohr says in a NEJM article earlier this year, it can be tempting to come to a relatively soothing conclusion:
The world has never before seen outbreaks of avian influenza on the scale of those that have swept through large parts of Asia, including densely populated China. From January through March 2004, more than 120 million poultry birds died or were destroyed as part of massive control efforts. Unprotected workers had intense exposures, as did health care workers. Virologic surveillance has demonstrated the concurrent circulation of human viruses. Hence, one conclusion is tempting: if H5N1 could reassort, it should have done so by now. The explanation may lie in sheer statistical luck. It could also be that reassortment has occurred but has resulted in viruses that are not viable, not pathogenic, or not more easily transmitted among humans than H5N1 currently is. If so, this news would be very good, and H5N1 could be moved a notch down on the watch list of viruses with the potential to cause a pandemic.
For what it’s worth, I accept that it’s difficult for H5N1 to reassort with human influenza virus, but I think that it would be foolish to presume that H5N1 will never find a way given enough opportunities.
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Henry Niman is on a jihad against the "H5N1 reassortment with human influenza" meme. While this may be motivated to some extent by his patent filings, I've followed his reporting on H5N1 for some time now, and he makes plenty of good points that stand on their own merits.
E.g.:
http://www.recombinomics.com/News/09300501/H5N1_Indonesia_Recombination_Vaccine.html
--quote--
Although WHO was notified of these changes in July of 2004, they continued to maintain that H5N1 was not in swine and continued to look for reassortment with human genes, which they continue to cite in press releases. H5N1 has never been reported to have human genes and mixing experiment with the 1997 H5N1 and human influenza failed to identify such changes. However, the lack of reassortment has been used repeatedly to offer reassurances, but is not reassuring because H5N1 has broadened its host range without reassortment with human genes. Such arguments are scientifically flawed, because the human receptor binding domain is on H and reassortment with a human H would not produce an H5 serotype.
Instead H5N1 evolves by recombination, and acquires mammalian polymorphisms to expand its host range to more mammals. These changes can be driven by selection, and a mismatched vaccine can drive such changes. Indonesia has been using a vaccination approach to control H5N1, but the program is not effective. H5N1 has become endemic in Indonesia and recent data indicates that the endemic H5N1 also is in asymptomatic swine and birds. Mild cases in humans suggest it may also be silently spreading in human populations also.
--end quote--
I like Dr. John Rutledge's post on his blog regarding the avian flu.
http://www.rutledgeblog.com/askrutl/archives/cat_policy.html#a000169