The present H5N1 bird flu virus strain seems to be more deadly for younger people, as was the case with the flu pandemic in 1918, says the World Health Organization (WHO). The average death rate for infected people is 56%, while for 10-19 year-olds it is 73%.
WHO says the chances of a pandemic occurring as a result of a H5N1 mutation is still significant, according to its report in its Weekly Epidemiological Record.
The report highlights the similarity between the death rates among young people today and 1918. In 1918, Spanish Flu had a much higher death rate among young people.
Normal human flu tends to be more deadly for the elderly and people with weakened immune systems. Normal human flu is estimated to kill up to half a million people every year worldwide.
The report looked at information gathered from over 200 confirmed H5N1 infection cases - all since 2003. Carefully following the epidemiology of all these and future human cases allows us to track the virus' evolution.
In some parts of the world H5N1 has established, and is now be classed as an epidemic among birds, including farmed and backyard poultry. This provides the environment for further human cases to occur, which gives H5N1 the opportunity to mutate and become more human transmissible.
The rate at which humans have become infected has gone up sharply this year. This is mainly because H5N1 is now present in Asia, Europe and Africa. Human infections have mainly happened as a result of close contact with infected poultry. There was a case in Indonesia where 7 family members all became infected and some of them most likely infected each other. However, they all shared a small room and slept together in it. H5N1 has always had the ability to spread from human to human if the sick person is in very close and continuous contact with another person - even in such cases, passing the infection on is rare.
In 2005 19 people died of bird flu infection during the first six months of the year. For the first six months of 2006 the number stands at 54 - 50% of these people were less than 20 years old, 90% were under 40. Human cases tend to peak during the winter months. It is expected that numbers will rise again toward the end of this year when temperatures go down in the Northern Hemisphere.
For the H5N1 virus strain to mutate it ideally needs to enter a human who is infected with the normal human flu virus. It could then exchange genetic information with the normal human flu virus and pick up its ability to easily spread from human-to-human.
One of the reasons H5N1 has such a high death rate for humans is that it only infects deep down in the lung. It does not, for the moment, infect the upper-respiratory tract. When patients start to feel symptoms the infection has already taken a grip. This is also why it is not easy for humans to become infected. A large cluster of viruses have to be continuously around a human for a long time so that some of them can eventually make their way deep down into the lungs and infect. The reason humans cannot infect other humans easily is because of this deep down infection. When an infected person coughs or sneezes very few viruses are expelled (they are too deep down).
For the virus to become easily human transmissible it will probably need to infect higher up, nearer the throat - the upper-respiratory tract. The good news is that upper-respiratory tract infections tend to be easier to treat. Hopefully, this means that when the virus does mutate, although it will spread among humans quickly, it will not be so deadly.
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