Evading Immunity: Drift
The H and N proteins of the influenza virus can undergo minor changes or mutations, leading to new strains. Scientists call this change "antigenic drift." Drift is an ongoing process and is one way the virus evades your body's immune system. Immunity to one strain of influenza virus does not necessarily confer immunity to a new strain which has undergone antigenic drift. This is one reason why yearly influenza vaccination is necessary to protect yourself against the newly circulating strain of influenza virus. Another reason for yearly influenza vaccination is that your body's vaccine induced antibody levels decrease a few months after vaccination, offering less protection against infection.
Evading Immunity: Shift
In contrast to the minor drift described above, major shifts in the influenza A viruses can also occur through other genetic mechanisms. These shifts can occur unpredictably and can lead to serious national epidemics or global epidemics called pandemics.
Many different types of animals, including humans, swine, birds, horses, aquatic mammals, and others, may become infected with influenza A viruses. Some influenza A viruses are unique to a particular type of animal and will not normally infect a different type of animal. However, some influenza A viruses may infect several different types of animals. Influenza A viruses which can infect birds (particularly migratory water fowl), swine, and humans are thought to be an important cause of major shifts in the influenza A virus. For example, suppose a swine becomes infected with an influenza A virus from a human and at the same time becomes infected with a different influenza A virus from a water fowl. When the two different viruses reproduce in the swine, the genes of the human strain and fowl strain may "mix," resulting in a new unique pandemic virus. This process is called genetic reassortment. The close proximity of humans, birds, and swine, such as may exist among farming communities in southern China , may enhance the possibility for this type of virus gene mixing.
(Animation: Genetic Reassortment Between Human and Nonhuman Influenza A Viruses)
