The extent to which a person’s immune protection is reduced against an antigenically drifted flu virus can vary, in part because different people may have different levels of pre-existing immunity against that flu virus. When a flu virus has become antigenically different (or “antigenically drifted”), this generally means that the virus’ antigenic properties are different enough that the body’s immune system (i.e., a person’s existing antibodies) will have a harder time recognizing and fighting against the virus. It also is possible for a single change in a particularly important location on the HA to result in a flu virus becoming antigenically different. However, the small changes in HA and NA that accumulate over time may result in viruses that are antigenically different, meaning a person’s antibodies bind differently or not at all to the virus, resulting in a loss or reduction in protection against that particular flu virus. This means that antibodies your immune system creates against one flu virus will likely recognize and respond to antigenically similar flu viruses (this is called “cross-protection”). Flu viruses that are closely related to each other usually have similar antigenic properties. The small genetic changes that occur in influenza viruses over time usually produce viruses that are closely related to one another, which can be illustrated by their location close together on a phylogenetic tree. The recombinant flu vaccine only targets the HA of the flu viruses it is formulated to protect against. For example, egg-based, cell-based, and live attenuated influenza vaccines (nasal spray) flu vaccines all target the HA and NA of the flu viruses they are formulated to protect against. Flu vaccines are designed to target one or more of the surface proteins/antigens of flu viruses. The changes associated with antigenic drift happen continually over time as flu viruses replicate (i.e., infect a host and make copies of themselves). The HA and NA surface proteins of influenza viruses are “antigens,” which means they are recognized by the immune system and are capable of triggering an immune response, including production of antibodies that can fight infection. One way flu viruses change is called “antigenic drift.” Drift consists of small changes (or mutations) in the genes of influenza viruses that can lead to changes in the surface proteins of the virus, HA (hemagglutinin) and NA (neuraminidase). Influenza (flu) viruses are constantly changing.

Define antigenic shift