Vaccine Mechanism
As the Centers for Disease Control and Prevention explains, vaccines work to stimulate the body's immune system. When a person gets infected by a virus, the body creates antibodies that are used to fight off that virus. Once the virus has been eliminated, the immune system makes special white blood cells called memory cells. These cells are primed to produce antibodies for the virus again if the body is exposed to it again. Vaccines work to stimulate the production of these memory cells by exposing the immune system either to dead forms of the virus or by specially modified viruses that cannot cause infections. By stimulating the production of memory cells, vaccines prepare the body to quickly fight off the virus in the future.
Flu Virus Mechanism
Medline notes that there are two different forms of the vaccine for influenza. The injectable form of the flu vaccine exposes the patient to a killed influenza virus. This means the virus can't cause an infection, although some patients experience a mild fever as the immune system gets activated by the vaccine. The flu vaccine can also be given as a nasal spray, which uses a weakened form of the virus. This type of vaccine helps prepare the lining of the nasal mucosa to fight off infections, which can keep the virus from getting into the bloodstream.
Vaccine Modification
Each year, scientists make new forms of the seasonal flu vaccine. This is because some forms of influenza (also known as type A influenza) mutate very rapidly. The memory cells that the flu vaccine creates only respond to the strain of the flu from which the vaccine was made. This means that as the flu virus mutates, older vaccines become less effective.
