Friday, December 23, 2005

Most reports on avian flu are simplistic

Most of the reporting on avian flu I’ve seen ignores important nuances. An article in the Cleveland Clinic Journal of Medicine provides a refreshingly sane treatment of the topic. Go read it in the original. I will outline a few underappreciated points here.

Though it’s widely known that there were three influenza pandemics in the twentieth century it’s less well appreciated that the 1918 pandemic was fundamentally different from the other two (1957 and 1968). The death count in the U.S. for the 1918 strain was an order of magnitude worse than the other two. The basis for this difference lies in the two distinct mechanisms by which the viruses alter their genetic makeup to enable efficient human to human transmission. One mechanism is co infection of a human and an avian strain in the same host with exchange of genetic material between the two strains. The resulting hybrid may have the ability for efficient human to human transmission but the virulence may be weaker than that of the pure avian strain. That’s what caused the 1957 and 1968 pandemics. In contrast, the 1918 pandemic apparently resulted from a mutation of the pure avian strain. The strain retained all or most of its virulence. These virulence factors have recently been characterized, and the deadly nature of the virus explained, with reconstruction of the genome.

So, there are many unanswered questions. When will the pandemic arrive? (Don’t forget, bird to human transmission of the bug was documented as early as 1997). It may be years away and once it occurs what type of pandemic will it be? It could result from a mutation of the pure avian virus producing a highly lethal pandemic or, as was the case in 1957 and 1968 it could be something much milder.

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