Antigenic drift

Antigenic drift is the gradual accumulation of mutations in viral genes encoding antigens, changing their structures and weakening recognition by existing antibodies and immune cells.

RNA virus mutation: Changes in RNA-virus genomes arise through copying errors and other mutational processes. Replication errors supply many of the mutations that can alter viral antigens.
Influenza A virus: A type of influenza virus that infects humans and other animals and can cause seasonal epidemics and pandemics. Its surface antigens drift continually, shaping seasonal immune escape.
Viral surveillance: The systematic collection and analysis of virus samples to track circulation, evolution, and disease patterns. Tracking circulating strains reveals which antigenic changes are spreading.
Genetic reassortment: The exchange of genome segments between viruses that infect the same cell. Reassortment can produce abrupt antigenic change, unlike drift's gradual mutation accumulation.
Hemagglutinin: A viral surface glycoprotein that binds host-cell receptors and helps influenza viruses enter cells. Mutations in this influenza surface protein can change antibody recognition.
Influenza B virus: A type of influenza virus that circulates mainly in humans and causes seasonal respiratory illness. Its lineages also drift, though they do not undergo antigenic shift.
Hemagglutination inhibition assay: A laboratory test that measures antibodies by their ability to block influenza hemagglutinin-mediated red-cell agglutination. Its results help detect reduced antibody recognition of drifted strains.
Antigenic variation: The alteration of pathogen antigens over time or during infection, reducing immune recognition. Drift is one evolutionary form of the broader phenomenon of antigenic variation.
Neuraminidase: An influenza surface enzyme that cleaves sialic acids and helps newly formed virions leave infected cells. Its antigenic sites also accumulate changes that affect immune recognition.
H3N2: An influenza A subtype defined by its hemagglutinin 3 and neuraminidase 2 surface proteins. H3N2 is a prominent example of rapid antigenic change complicating vaccine matching.

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