Influenza viruses are changing continuously and new strains emerge from time to time. If a new influenza virus subtype, that is very different from all previous viruses, emerges and causes disease in the human population, people will have little or no specific immunity to it, and consequently this virus may spread quickly and become a worldwide threat.
Antigenic drift is caused by small changes in the virus surface proteins that happen continually over time and allow the virus to escape from antibodies that were generated in the population upon infection with previous seasonal influenza viruses. Each year, the virus strains represented in the seasonal influenza vaccine are updated to keep up with changes in the circulating influenza viruses.
Another type of change – of even greater public health concern – is called antigenic shift. Antigenic shift is an abrupt, major change in the circulating influenza A virus subtype. This may happen when an avian and a mammalian (human) influenza A virus simultaneously infect one mammalian host – e.g. a pig or a human being – and their genome segments are shuffled like a deck of cards. This results in a virus with a new combination of genome segments. If the surface proteins (H and N) are now of avian origin and have not been ‘seen’ by the human population before, there will be little or no antibodies against such a hybrid human-avian virus in the human population. Consequently, the resulting new influenza A virus may start spreading in the human population, with devastating consequences: a pandemic outbreak of influenza.
Another way in which an antigenic shift may arise is through a process of sequential adaptive mutation of a virus that repeatedly crosses the species barrier from birds to mammals. This may lead to the gradual adaptation of the avian virus to the mammalian species, allowing it to spread efficiently from mammal to mammal. This may happen directly in humans, or in another mammalian species such as the pig or the cat, from which it is then transmitted to humans. Unlike seasonal influenza, a new pandemic virus can show up any time when the above-mentioned events take place. However, the most favourable conditions for its spread in moderate climate zones are, like for seasonal influenza, present during the winter months. The resulting wave of illness and associated mortality can then spread rapidly across the globe, affecting many millions of people.
The WHO criteria for a pandemic are:
- The virus is new for humans and limited or no immunity is present in the human population (1 and 2)
- The new virus has already infected humans (1 and 2)
- The virus is highly transmissible from human to human (2)
(1): Conditions met by the HPAI-H5N1 virus
(2): Conditions met by the H1N1 Mexican flu virus
The World Health Organization has defined different alert phases on the way to an influenza pandemic.