It turns out the flu virus is unique in its ability to mutate and adapt, making it impossible to use a single vaccine to combat it. The situation is complicated by the fact that animals, both wild and domestic, are a sort of "mixing bowl" in which new strains of the virus are created.
The variation in the virulence of the flu is apparently due to the varying degrees to which our immune system recognizes the two types of proteins on the surface of the virus -hemagglutinin (H) and neuraminidase (N).
The virus strain is said to undergo "drift" when there are minor changes in the two proteins. This is what happens most years. But sometimes it undergoes a more major change, called "shift," which is due to reassortment of the viral genes, often when it is mixed with another strain in animals. This is a really bad thing because it means humans will have no antibodies that recognize the new strain, and so no immunity to it.
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Posted by ellen at January 15, 2004 10:54 PM Flu can also jump from species to species without reassortment. For example the chicken flu that has jumped to humans in several cases recently. However it usually will not be easily transmitted between humans on first "jump." The danger is that someone will catch it that already has human flu virus in their cells, and then the avian virus can swap genes with the human version, and WILL be easily transmissable. This means it will be pretty much unknown to human immune systems, but have all the transmittability of the human varieties. Scientists think this may have been what happened in 1918-1919 pandemic swept the world causing the deaths of 20 million people.
As it is, the flu yearly causes the deaths of 20,000 to 40,000 people in the United States alone, making one wonder why it is considered just a "common ailment" instead of a scourge. Only recently have the US recommendations been for everyone to take the flu vaccine, and that was because they knew they were dealing with a particularly bad variety this year. Ironically, they recommended the vaccine to all in a year when it was unusually ineffective.
I have two friends, both in their late 30's, that nearly died in recent years of complications of the flu. Both of them were in good health otherwise, contradicting the usual idea that the flu only kills people who are old or weak. I find it very strange that the FDA and CDC and other such bodies worldwide have a relatively casual attitude toward a disease proven to be so dangerous, even in its less virulent years.
Additional reference:
The genetic genesis of a killer flu - From Science - How animal RNA makes human viruses worse
CDC Morbidity and Mortality Weekly Report: Preliminary Assessment of the Effectiveness of the 2003--04 Inactivated Influenza Vaccine --- Colorado, December 2003
Statistics about the Flu
Barnyard Blues
Thinking about skipping your flu shot this year?
Influenza, an Ever-evolving Disease, by Robert Heckert
