From one year to another there is usually a slight change in the circulating influenza virus caused mostly by mutations in the genetic makeup of the virus. This slight change is referred to as a drift. Because of exposures to influenza viruses over previous years as well as the protection afforded by vaccinations, most of the world has some degree of what is known as background immunity to influenza. This at least partially protects us from actual infection or from serious disease caused by the infection. This is the reason we do not have a global catastrophic outbreak of influenza every year.
