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Why do I have to get a flu shot every year?

Influenza is commonly called "the flu" and is caused by a variety of related Influenza viruses. Most people infected with the flu virus recover in one to two weeks, but the flu should not be taken lightly as some people can develop serious and potentially life threatening medical complications, such as pneumonia. In an average year, the CDC reports about 90 million cases of the flu and there are on average about 36,000 deaths/year in the United States and many more people are hospitalized because of it. Elderly people and people with chronic health problems and the very young tend to be most susceptible. Therefore, protection from flu virus is important and the most effective way to protect yourself is by vaccination every year.

Humans can be infected with influenza virus types A, B, and C. Influenza A is the one we worry most about and it is maintained in nature in wild birds, but can infect pigs, horses and many other hosts. The genetic information of Influenza virus is distributed among eight different segments or molecules. When different flu viruses infect the same host, these pieces of genetic information can mix and match to create populations of entirely new flu viruses with varying characteristics. A bit like shuffling cards. The type of genetic information used by the flu viruses and the ways in which these pieces of genetic information are copied to generate virus progeny also introduces random mutations into these pieces of viral genetic information which will further increase the diversity of progeny virus. In Asia the density and intermixing of pigs and birds on farms leads to the mixing of different types of flu viruses. The genetically mixed populations of flu viruses represent the starting points for new flu epidemics that can spread around the world. This is where scientists go and look to determine which flu viruses are going to emerge every year to design the next year's flu vaccine. The immunity you have from previous flu infections or vaccinations will be only partially protective against these newly arising viruses so you need a new flu vaccination every year.

Each year, 9-10 months in advance of "flu season," the U.S. Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee predicts which of three types of influenza viruses are most likely to occur, and then instructs vaccine manufacturers what is the appropriate vaccine be developed to combat it. Since it takes one to two weeks after the vaccination for the protection to develop, the best time to get this vaccination is during October through mid-November when people are usually relatively healthy and have not yet been exposed to the flu. Make no mistake; the best way to prevent the flu is the vaccine.

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