Influenza viruses are constantly changing. They can change in two different ways:
One way they change is called “antigenic drift.” These are small changes in the genes of influenza viruses that happen continually over time as the virus replicates. But these small genetic changes can accumulate over time and result in viruses that are look different to the body’s immune system. When this happens, the body’s immune system may not recognize those viruses. That is why the flu vaccine composition must be reviewed each year and updated as needed to keep up with changing viruses.
The other type of change is called “antigenic shift.” Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in a completely different virus that infects humans. Such a shift occurred in the spring of 2009, when an H1N1 virus with a new combination of genes emerged to infect people and quickly spread, causing a pandemic. When shift happens, most people have little or no protection against the new virus.
While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.