Virus genomes: shift and drift
The influenza genome is in constant flux
Reassortment refers to the mixing of RNA in the virus genome, which can occur if more than one type of influenza virus infects a single cell, as seen in the emergence of influenza A(H1N1)pdm09, which was from a mix of different ancestors.
In effect, influenza viruses have an enormous global pool of genetic material that can come together in different combinations. Changes in the genome by reassortment may change significantly the proteins on the surface of the virus, a process known as ‘antigenic shift’.
Mutations are smaller-scale changes that occur in the genetic material of an individual virus. However, these mutations can lead to changes in the surface-protein-encoding genes, resulting in new variants through the process of ‘antigenic drift’. Changes may arise because of selective pressures (e.g. from the host’s immune system) or the accumulation of random changes over time.
Shifts can be the earthquakes that unleash new pandemics, but drift is a constant nuisance, as new variants evade immune responses and render vaccines ineffective within a year or so.Lead image:
Jer Thorp/Flickr CC BY