Antigenic drift is the slow accumulation of small mutations in a virus that accumulate over time as the virus replicates.
A person infected with a particular virus develops antibodies against that virus.
As the small mutations accumulate, the antibodies created against the older virus eventually will not recognize the “newer” virus.
The most common example is the flu virus.
Each year’s flu virus contains three strains of the virus.
After vaccination, the immune system creates antibodies that protect against these strains.
If the virus mutates too much, the antibodies are not able to recognize its antigens, and it will again be able to infect the cells.
This flow chart from the National Institute of Allergy and Infectious Diseases illustrates the process.