Antigenic drift is the gradual accumulation of mutations in influenza surface proteins, allowing immune escape.
Mechanism
- Point mutations in hemagglutinin (HA) and neuraminidase (NA)
- Changes epitopes that antibodies recognize
- Previous immunity becomes less effective
Epidemiological Patterns
- Annual epidemics from drifted strains
- Gradual replacement of old strains by new variants
- Cross-immunity depends on antigenic distance
Why Annual Vaccines?
Drift means last year's vaccine may not match this year's circulating strain. WHO updates vaccine formulation annually.