“Two particular features of the influenza virus contribute to its rapid evolutionary capacity to change its antigens and escape immune recognition,” McCaughey explained.
The first is a high rate of errors during replication of the virus genetic material, causing rapid mutation. The second is the separation of the RNA into eight separate segments.
When two different viruses infect the same cell, segments can be swapped. This is equivalent to “sexual reproduction in viruses”.
“Most influenza genetic diversity is in birds,” McCaughey added. Influenza A infects wild birds, domestic birds, and swine, as well as humans.
Most seasonal influenza epidemics are caused by virus strains similar to those of previous years, so we have some immunity. When a new virus jumps from an animal host to humans, we are more susceptible to widespread infection.