## Influenza Antigenic Variation and Epidemiology **Key Point:** Influenza B virus **does NOT undergo antigenic shift** because it infects only humans and some marine mammals—reassortment requires co-circulation of multiple subtypes in a shared host, which does not occur with influenza B. ### Antigenic Drift vs. Antigenic Shift | Mechanism | Cause | Frequency | Outcome | |-----------|-------|-----------|----------| | **Antigenic Drift** | Point mutations in HA/NA genes (RNA polymerase lacks proofreading) | Continuous; annual | Seasonal epidemics; vaccine updates needed | | **Antigenic Shift** | Reassortment of genome segments (requires co-infection of 2+ subtypes) | Rare; every 10–40 years | Pandemic potential; new subtype emerges | ### Influenza A vs. Influenza B | Feature | Influenza A | Influenza B | |---------|-------------|-------------| | **Host range** | Humans, birds, pigs, horses, marine mammals | Humans, seals (rare) | | **Subtypes** | Multiple (H1N1, H3N2, H5N1, H7N9, etc.) | No subtypes; only lineages (Victoria, Yamagata) | | **Antigenic shift** | Yes (reassortment in mixed-host settings) | **No** — only single host circulation | | **Pandemic potential** | High | Low (only seasonal epidemics) | | **Genetic diversity** | High | Lower | **High-Yield:** Only **Influenza A** has caused pandemics (1918 H1N1, 1957 H2N2, 1968 H3N2, 2009 H1N1). Influenza B causes only seasonal epidemics. **Clinical Pearl:** In India, both H1N1 and H3N2 co-circulate seasonally, particularly during winter months (December–February). Rapid antigen tests and RT-PCR are used for diagnosis; treatment with oseltamivir (neuraminidase inhibitor) is most effective within 48 hours of symptom onset. **Mnemonic:** **SHIFT = Segmental reassortment in mixed Host Infection Facilitates pandemics** — but only in Influenza A. [cite:Harrison 21e Ch 195]
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