Breakthrough infection
When a person is vaccinated, their immune system develops antibodies that recognize specific segments of  viruses or viral-induced proteins. Over time, however, viruses accumulate genetic mutations which changes the structure of viral proteins. If these mutations occur in sites that are recognized by antibodies, the mutations block antibody binding, which inhibits the immune response. This phenomenon is called antigenic drift.  Breakthrough infections of Hepatitis B and mumps are partially attributed to antigenic drift.