The two new mechanisms of change in influenza strains are antigenic drift and antigenic shift
However, every decade or so a more virulent strain may arise due to antigenic shift
(associated with a reassortment of genes coding for the HA and NA surface proteins), leading to sustained escape from acquired immunity to influenza present in the general population and resulting in a more profound pathology with high case fatality rates.
The abrupt changes in influenza A viruses commonly arise when genome segments reassort, sometimes acquiring new surface protein genes from animal sources, leading to antigenic shift
as was seen in the recent influenza A(H1N1) pandemic strain (5).
: Antigenic shift
is a more abrupt alteration in HA or NA in the influenza virus.
These strains occur because of the phenomenon known as antigenic shift
, in which humans are infected with avian influenza viruses or viruses that contain a combination of genes from human and avian sources.
The influenza virus has the capacity for antigenic shift
through its ability to reassert with other influenza virus strains.
Explaining how the virus can mutate, Tudor said: "The mutation, or antigenic shift
, would occur in a cell when it is infected with two different strains of the H1N1 virus," says Tudor.
Pandemics, or world-wide epidemics, occur when antigenic shift
causes the sudden and unpredictable emergence of a new human influenza virus to which most of the population is susceptible.
This strategy, known as antigenic shift
, works well as a long-term survival tactic: immunologically, a new virus subtype starts from scratch and is guaranteed a very large population of susceptible hosts.
But occasionally, influenza A viruses undergo formation of an entirely new subtype through a process called antigenic shift
, resulting in a new H or N protein, which creates the possibility of a pandemic.
Scientists can usually stay ahead of the mutations and create a new vaccine each year to help prevent a flu caused by the strains that mutated gradually, but in rare cases when one of the type A strains mutates abruptly and unexpectedly in a process called antigenic shift
, that year's vaccine can't prevent an outbreak.