When antigenic drift occurs, the body’s immune system may not recognize and prevent sickness caused by the newer flu viruses. It also is possible for a single change in a particularly important location on the HA to result in antigenic drift. However, the small changes associated with antigenic drift can accumulate over time and result in viruses that are antigenically different (further away on the phylogenetic tree). 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 changes that occur from antigenic drift usually produce viruses that are closely related to one another, which can be illustrated by their location close together on a phylogenetic tree. The nasal spray flu vaccine (LAIV) may target both the HA and NA of a flu virus. Most flu shots are designed to target the HA 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 block 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.
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