The genes of an influenza virus that determine the haemagglutinin and neuraminidase antigens slowly change over time (known as ‘antigenic drift’) by mistakes made when the virus reproduces and copies its genetic material. These changes mean that immunity gained from previous infection or immunisation becomes less effective or ineffective, explaining why a previous bout of seasonal influenza does not prevent subsequent infections, and why we need a new seasonal influenza vaccine each year.
Under the right circumstances, a new subtype of influenza can emerge. When this type of major change in the influenza virus occurs, it is known as ‘antigenic shift’. Because the immune system has never been exposed to viruses of this new subtype before, and has no ability to recognise it and combat it, the virus tends to cause very severe infections with a high mortality rate.
Antigenic shift (major change in the virus) can occur in one of two ways:
- reassortment, which is the mixing of the genes from an animal or bird virus and a human influenza virus—this is thought to be able to occur if an animal or human is infected with both viruses at the same time
- adaptive mutation, which is change in the genes of an animal influenza virus, which may allow the virus to infect and be transmitted easily between humans.
- Only strains within the subtypes H1, H2 and H3 have been known to have adapted successfully to transmit easily between humans. When they first emerged, these subtypes initially caused pandemics but then, over the course of a number of years, adapted to cause seasonal influenza. Only the H1 and H3 viruses are currently circulating in humans.
- Other subtypes (such as H5, H7 and H9) have caused human infections, but have not developed the capacity to be transmitted easily and rapidly between humans.
- The remaining subtypes, which cause infection in birds and some animals, have not developed the ability to infect humans yet, but they are closely monitored by scientists.
The WHO has studied the development of previous pandemics in detail and stated that the next pandemic is likely to develop by moving through the following steps:
- First, an influenza virus in birds or animals develops the ability to be able to infect humans and cause serious disease. During this initial phase, although the virus can cause disease in humans, the virus is unable to transmit efficiently between humans. Contact with infected animals is needed for human infection to occur.
- Following further genetic change, the virus may become more efficient at passing from human to human, first within small groups or clusters (families or community networks) and later over wider but still localised areas.
- Finally, the virus is able to transmit readily between humans. It spreads rapidly because of the short incubation period and the infectious nature of influenza. Rapid global spread is aided by the large amount of international travel that takes place every day between virtually every country in the world.