Viruses and Immunity
Viruses are pathogenic organisms made up of genetic material and proteins that infiltrate the host cells and use the resources and machinery of the cells to make more copies of the virus. The body fights viruses using special proteins called antibodies. Antibodies are made by the immune system and they bind specifically to parts of viruses or to virus infested cells. When an antibody binds to its target, it activates the immune system to attack the target. Antibodies are the main way in which the body can fight viruses and if the body has antibodies for a given virus, the immune system can quickly and efficiently fight off the viral infection.
Flu Shot Mechanism
Flu vaccinations can come in two forms. One of the forms is a nasal spray, and the other is an injection. Both forms work by stimulating the immune system to make antibodies against the flu. Flu vaccines work by presenting the body with a form of the flu virus that has been specially modified so that it cannot cause infections. The immune system, however, still recognizes it as a virus and makes antibodies against it. These antibodies are then effective at fighting off infections from "live" flu viruses. As a result, flu shots either greatly reduce the severity of seasonal flu infections or prevent people from developing an infection in the first place.
Antigenic Drift
As the Center For Disease Control explains, the flu virus is constantly mutating and changing, which is a phenomenon known as antigenic drift. This means that each year there are new strains of the influenza virus. Antibodies for one strain of the virus may not create immunity to newer forms. For this reason, each year the flu shot is changed to account for new influenza strains. Before flu season, scientists determine which strains of the flu are likely to be prevalent that year and make a new vaccine to treat these kinds of flu. Thus, scientists must develop new flu shots each year.
