Most of the vaccinations we get last for many years and some we only get as children. So it makes sense that people often wonder why they need an influenza vaccination every fall. The reason is because of the virus’ high mutation rate. There are three types of influenza viruses: A, B and C. Within these three types, there are two types of mutations. One mutation creates different subtypes and the other creates completely new strains.
The first type of mutation is known as antigenic drift. The influenza virus contains capsids that hold its genetic material. Capsids are made of several hundred types of glycoproteins known as hemagglutinin (HA) and neuraminidase (NA). These proteins act as antigens that tell the immune system the virus is foreign. Once the virus infects cells, the genes for these two proteins are replicated, or copied. A lot of errors are made during that process that lead to mutations in the copies. These mutations create new HA and NA subtypes that are new antigens that have to be recognized as foreign by the immune system.
The second type of mutation is antigenic shift, which only occurs with Influenza A because of its ability to infect human and non-human animals. The premise lies in the fact that the flu virus is constantly evolving it outer protein shells. The two proteins that make up the shells are hemagglutinin and neuraminidase ( H and N). All of the genes that code for the influenza virus are “segmented” which means that whole genes can be moved around. If two or more strains of influenza that infect humans and non-humans are together in a cell, the HA and NA genes can move between the strains and create a completely different one. If this strain has not been seen in most hosts an outbreak on a global scale, known as a pandemic, may occur.
These are the two reasons why people who have had the flu before can get it again, making an annual flu vaccine necessary.
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Last updated October 2013