An antigenic drift is a natural evolutionary change in the composition of antigens on the outside of a virus. This phenomenon has been well documented with the influenza virus, as antigenic drift forces scientists to develop new flu vaccines every year to protect people against newly emerged strains of the virus. It is different from antigenic shift, a sudden and radical change in the composition of a virus where two different subtypes combine into a new form, as seen with variations like avian influenza.
Viruses, like other organisms, are constantly mutating. With each reproduction, small changes occur. Some of these changes are beneficial, allowing the viruses to survive, while others may be deleterious, leading those viruses to die out. With antigenic drift, the antibodies that coat the outside of the virus change. When organisms with immunity to earlier evolutions of the virus are exposed, their immune systems may not identify the changed antigens, allowing the virus to colonize the body and infect people who come into contact with the patient.
Some viruses mutate slowly, while others do so more rapidly. Mutation in influenza viruses has been closely studied by researchers interested in seasonal flus and the development of effective vaccines. Every year, a new vaccine with a combination of antigens is prepared and administered to people at risk of developing complications if they get the flu, as well as to members of the general population who are worried about influenza.
Researchers can track viral evolution by following antigenic drift over time, and flu tends to follow distinct patterns, returning to earlier arrangements of antibodies as it mutates. This can allow researchers to partially predict changes caused by antigenic drift in the influenza virus while preparing for the next flu season. Changes in the expected evolution of flu viruses can raise a red flag, suggesting that something is pressuring the viral population and causing it to behave abnormally.
In antigenic shift, something more radical happens. Instead of mutating naturally as it reproduces, a virus combines with another subtype to create a new virus. For example, a virus native to pigs can combine with a human influenza virus, creating a form of influenza that will infect humans with viruses that have a set of antibodies their immune systems cannot recognize and do not fight. This can result in pandemic influenza, where numerous people get sick as the virus quickly spreads and people can die as a result of their flu infections.
