Between primary and secondary infections, or following vaccination, a virus may undergo antigenic drift, in which the viral surface proteins (the epitopes) are altered through natural mutation, allowing the virus to escape the immune system. When this happens, the altered virus preferentially reactivates previously activated high-affinity memory B cells and spur antibody production. However, the antibodies produced by these B cells generally ineffectively bind to the altered epitopes. In addition, these antibodies inhibit the activation of higher-affinity naive B cells that "would" be able to make more effective antibodies to the second virus. This leads to a less effective immune response and recurrent infections may take longer to clear.