1.2 What causes pandemic flu?
1.2.1 How flu viruses change
'Some of the commonest infections have a particular ability to change, influenza viruses being the chameleons of the microbial world.'
Getting ahead of the curve - a strategy for combating infectious diseases
A report by the Chief Medical Officer, January 2002
Flu viruses have a particular characteristic that enables them to cause annual epidemics and even pandemics: change. Type A viruses undergo frequent changes in their surface antigens or proteins. These changes can be minor - known as antigenic drift - or major, known as antigenic shift.
Antigenic drift: 'ordinary' flu
Antigenic drift: ordinary flu
Antigenic drift occurs constantly among influenza A viruses resulting in the emergence of new strains every year. These new strains cause the annual flu epidemics we experience each winter. Some annual flu epidemics are worse than others. This happens when the new strains are significantly different from previous strains. The more a strain differs from previous ones, the less immunity a population will have to it.
Antigenic shift: pandemic flu
Antigenic shift: pandemic flu
Occasionally major changes occur in the surface antigens (proteins) of influenza A viruses. These changes are much more significant than those associated with antigenic drift. Such changes can lead to the emergence of a pandemic strain by creating a virus that is different from recently circulating strains. The population would have very little or no immunity to it since they will not have been infected with it or vaccinated against it before. This lack of immunity allows the virus to spread more rapidly and more widely than an ordinary flu virus.
How does antigenic shift occur?
How does antigenic shift occur?
Antigenic shift usually occurs in two ways: either as a sudden adaptive change during replication of a normal virus, or from an exchange of genes between a human strain of an influenza A virus and an animal strain. This genetic exchange or re-assortment produces a new virus capable of causing a pandemic in humans. Genetic exchange occurs when an animal becomes infected with a human and an animal flu virus at the same time so-called co-infection. The animal in which this genetic exchange takes place is often described as a mixing vessel. The domestic pig is a likely mixing vessel because it is susceptible to both human and avian (bird) flu. However, more recently experts fear that people may also serve as mixing vessels (see Fig 1.1).
This possibility that people could act as 'mixing vessels' has caused particular concern in the context of the highly pathogenic avian flu (A/H5N1) currently circulating in Asia. This strain of avian flu has demonstrated the ability to infect people. Experts fear that people infected with avian flu could also become infected with a human flu strain at the same time, allowing the exchange of genes that could lead to the emergence of a pandemic strain. Alternatively, the avian flu strain could evolve into a pandemic strain simply by re-adapting to the human body, thereby acquiring the ability to pass easily from person to person.
The role of avian flu in the emergence of pandemic flu is described in more detail in Chapter 2.
