Antigenic drift in influenza viruses is caused by which mechanism?

Explanation

## Antigenic Drift vs. Antigenic Shift in Influenza **Key Point:** Antigenic drift is the gradual accumulation of point mutations in the genes encoding hemagglutinin (HA) and neuraminidase (NA) surface proteins. This leads to small, incremental changes in viral antigenicity over time. ### Antigenic Drift: Mechanism 1. **RNA-dependent RNA polymerase errors** — Influenza virus lacks proofreading mechanisms 2. **Point mutations accumulate** in HA and NA genes at a rate of ~1 mutation per 1000 nucleotides per replication cycle 3. **Immune escape** — Mutations allow virus to evade previously acquired antibodies 4. **Seasonal epidemics** — Drift causes yearly influenza outbreaks requiring annual vaccine updates ### Antigenic Drift vs. Antigenic Shift | Feature | Antigenic Drift | Antigenic Shift | | --- | --- | --- | | **Mechanism** | Point mutations in HA/NA genes | Reassortment of gene segments | | **Rate of change** | Gradual (yearly) | Sudden (abrupt) | | **Frequency** | Continuous | Occasional (every 10–40 years) | | **Severity** | Seasonal epidemics | Potential pandemics | | **Requires** | Single virus strain | Co-infection of 2+ strains | | **Example** | H1N1 seasonal variants | 1918 Spanish flu, 1957 Asian flu, 1968 Hong Kong flu, 2009 H1N1 pandemic | **High-Yield:** Antigenic drift is why: - Influenza vaccine must be reformulated annually - People can be infected with influenza multiple times in a lifetime - Seasonal flu strains change every year **Mnemonic:** **DRIFT = Drift is Recurring, Incremental, Frequent, Tiny changes** **Clinical Pearl:** Antigenic shift is the more dangerous process because it can produce entirely novel viral subtypes to which the population has no immunity, leading to pandemic spread. [cite:Robbins 10e Ch 8]