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H5N1 Pandemic Potential
November 18, 2004
>>Between a quarter and a third of the world's population will fall ill, according to new World Health Organization estimates, and 1 per cent of the sick will die.<<
A 1% case fatality rate for is a VERY optimistic number. Most of the current worry centers on H5N1 and the case fatality rate in Asia is in the 70-80% range, which would put human fatalities well north of 1 Billion. There really is little support for this rate to fall to 1% as the virus acquires human to human transmission. Although much has been made of re-assortments emerging from dual infections, the virus really evolves via recombination. Acquiring human receptor binding sequences via recombination would not be difficult and would involve a very small portion of the H gene. Such a recombinant could achieve efficient human to human transmission without significant change in the rest of H and no change in the other genes. Thus maintaining a case fatality rate of 70-80% would not be difficult.
Many factor favor such a scenario. The current case fatality rate is the highest recorded for human influenza infections. Those who have died have been children and young adults, indicating that the virus is not well recognized by human immune recognition systems. H9N2 viruses with H5N1 genes have been reported, as have H5N1 viruses with H9N2 genes. Since some H9N2 viruses already have receptors that target humans, creation of recombinants would not require mammalian infections.
The number of reported H5N1 infections in birds was at an all time high last season and recent lab experiments show that the H5N1 isolates that are lethal in humans in Vietnam and Thailand can grow to high titers in duck intestines without causing disease in the ducks. A second infection in these asymptomatic ducks can lead to recombinants that are shed in large amounts.
Thus, the migration of birds provide new flu sequences for the creation of novel recombinants which could retain the high case mortality rate seen in 2004 coupled with the ability to efficiently transmit from human to human.