A trainwreck in slow motion
Dr. Bernd Sebastian Kamps,.Senior Editor at Influenza Report 2006 has taken a look at the age distribution of 116 WHO confirmed H5N1 cases. Bar charts can be seen at the link. Some additional age breakdowns stratified by country were made available to ProMed by Dr. Kamps.
The age distribution shows an extreme skew toward the younger age groups. In ordinary (seasonal, inter-pandemic) influenza, it is the oldest age groups that have the highest mortality. But these data show half the cases below the age of 16, three quarters below the age of 29 and 90% below the age of 39. Mean age is about 20 years old. The mean age of cases in Turkey appear to be somewhat lower (13 years old).
This is an extraordinary distribution and not matched by any influenza outbreak on record to our knowledge. A characteristic of pandemic influenza is a marked shift to younger age groups, but this is more extreme than anything yet seen. In 1918 most of the excess mortality was under the age of 65, but there continued to be considerable "normal" influenza mortality in the over 65 age group.
Not so, as far as we know, in this case. It may be we are seeing some kind of reporting bias, i.e., that younger age cases are being recognized but not older ones, although this seems unlikely given the severity of the illness. It is also possible that there is something about exposure that is higher in the young. It has been suggested that it is the youngest who are closest to poultry, either by virtue of having the chore of collecting the eggs in the henhouse in the morning or because they play with the birds as pets. But it has been a puzzling feature of H5N1 outbreaks that adult poultry workers and those involved in culling operations rarely become ill or show signs of infection. It is possible there is some kind of cross immunity in the older age groups or other unidentified biological feature that either protects an older person or makes the younger ones especially susceptible.
As yet this is not a pandemic virus. But it has the hallmarks of one in the making. It is like watching a trainwreck in slow motion.
The age distribution shows an extreme skew toward the younger age groups. In ordinary (seasonal, inter-pandemic) influenza, it is the oldest age groups that have the highest mortality. But these data show half the cases below the age of 16, three quarters below the age of 29 and 90% below the age of 39. Mean age is about 20 years old. The mean age of cases in Turkey appear to be somewhat lower (13 years old).
This is an extraordinary distribution and not matched by any influenza outbreak on record to our knowledge. A characteristic of pandemic influenza is a marked shift to younger age groups, but this is more extreme than anything yet seen. In 1918 most of the excess mortality was under the age of 65, but there continued to be considerable "normal" influenza mortality in the over 65 age group.
Not so, as far as we know, in this case. It may be we are seeing some kind of reporting bias, i.e., that younger age cases are being recognized but not older ones, although this seems unlikely given the severity of the illness. It is also possible that there is something about exposure that is higher in the young. It has been suggested that it is the youngest who are closest to poultry, either by virtue of having the chore of collecting the eggs in the henhouse in the morning or because they play with the birds as pets. But it has been a puzzling feature of H5N1 outbreaks that adult poultry workers and those involved in culling operations rarely become ill or show signs of infection. It is possible there is some kind of cross immunity in the older age groups or other unidentified biological feature that either protects an older person or makes the younger ones especially susceptible.
As yet this is not a pandemic virus. But it has the hallmarks of one in the making. It is like watching a trainwreck in slow motion.
posted by Revere at
2/12/2006 08:42:00 AM
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