Human Mortality From H5N1 - Demographic Characteristics

Demographic Characteristics

Review of patient ages and outcomes reveals that H5N1 attacks are especially lethal in pre-adults and young adults, while older victims tend to have milder attacks and to survive. This is consistent with the frequent development of a cytokine storm in the afflicted. Few persons over 50 years of age seem to have become infected by H5N1, and very few have died after suffering an H5N1 attack. Instead, the age-fatality curve of H5N1 influenza attacks in humans resembles that of the 1918 Spanish pandemic flu, and is the opposite of the mortality curve of seasonal flu strains, since seasonal influenza preferentially kills the elderly and does not kill by cytokine storm. An additional factor which may be active is that H1N1 was the predominate human flu circulating from 1918 until 1957 when the H2N2 strain emerged. Hence those over 50 years old have had the opportunity to be exposed to H1N1, and to develop some immune response to the N1 group contained in that human form of flu. Likewise, annual flu vaccination includes inoculation against a type-A human H1N1 flu, leading to the possibility that the annual flu shot or Flumist inoculation might confer some immunity against H5N1 bird flu infection, and indeed testing the blood of volunteers to look for immune response to H5N1 found that some blood samples showed immunity, but more of the blood samples of persons who had received the flu shot showed an immune response.

Another factor complicating any attempt to predict lethality of an eventual pandemic strain is the variability of the resistance of human victims to the pathogen. Many human victims of the current H5N1 influenza have been blood relatives (but rarely spouses) of other victims. Though this observation seemed to suggest that a familial genetic susceptibility might have played a role in human infection, a study by researchers at the Harvard School of public health noted no significant familial pattern of infection. Clearly, those whose immune systems are best able to fight off the virus are the most likely to survive a pandemic. Those with impairment of the needed immune function, whether from familial genetics or from AIDS, have poorer chances. Moreover, the health care system is generally expected to be overwhelmed throughout a pandemic. Persons needing access to medical care, whether for influenza or for unrelated serious maladies, are unlikely to receive the accustomed care, and without it their survival chances will be reduced.