Influenza Viruses

Antigenic Drift and Shift

The influenza virus is a very simple genetic structure with its genes in eight segments of single stranded RNA.

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RNA replication leads to increased susceptibility to mutations. Segmentation allows 'gene-swapping' (or re-assortment) during mixed infections between different sub types of influenza viruses, and the emergence of a pandemic virus. Mutation and re-assortment are the genetic basis for the two types of changes: antigenic drift and shift.

Antigenic drift: is constantly occurring in both types A and B viruses. The HA and/or NA of the new strain are sufficiently different to evade (at least partially) the pre-existing human immunity. This leads to the seasonal epidemics.

Antigenic shift: occurs only in type A virus. It describes the emergence of an entirely new virus sub type. When this new sub type emerges, it causes a pandemic because there is no pre-existing immunity in humans.