All influenza (flu) viruses have a relatively high mutation rate, however, scientists are particularly concerned about the highly pathogenic avian influenza (HPAI) subtype HPAI H5N1. Avian influenza, in the presence of human or seasonal flu, has the potential to mutate and develop into a new subtype of virus. Globally, there would be little immunity to this new strain and it could have significant public health consequences. Influenza viruses mutate in two ways; antigenic drift and antigenic shift.
- Antigenic Drift refers to the process of small genetic changes in the virus that happen gradually over time. Antigenic drift produces new virus strains that may not be recognized by the body's immune system. It is this reason that a new seasonal flu vaccine is developed each year and incorporates up to three different viral strains that researchers conclude will be most common during the upcoming season.
- Antigenic Shift refers to an abrupt and substantial change to produce a new influenza A virus subtype in humans that was not currently circulating among people. Antigenic shift can occur either through direct animal (poultry)-to-human transmission or through mixing of human influenza A and animal influenza A virus genes to create a new human influenza A subtype virus through a process called genetic reassortment.
Pandemic
At this point, there is no evidence of genetic reassortment between avian H5N1 viruses and human viruses; however, if H5N1 continues to circulate widely among poultry, the potential for emergence of a reassorted pandemic strain remains a concern. A global influenza pandemic (worldwide spread) may occur if three conditions are met:
- A new subtype of influenza A virus is introduced into the human population.
- The virus causes serious illness in humans.
- The virus can spread easily from person to person in a sustained manner.
While the virus is not able to spread easily from person to person, it does cause severe disease. WHO places the current pandemic phase level at a 3, "no or very limited human to human transmission." However, the case fatality rate of HPAI H5N1 is 60%. In other words, 60% of those infected with HPAI H5N1 die. The potential for a mutation to increase the transmission rate is low; however, the implications it would have on public health would be great.
For more information of pandemic response plans visit pandemicflu.gov
