Antigenic shift is the mechanism whereby new human pandemic strains arise. Pandemic strains are characterized by their sudden emergence in the population and their antigenic novelty. During the twentieth century, three pandemics occurred; in 1918 the causative strain was H1N1, in 1957 H2N2 and in 1968 H3N2.

1. As the influenza virus genome is segmented, it is possible for two influenza strains to exchange their genes upon co-infection of a single host, for example swine, leading to the construction of a replication-competent progeny carrying genetic information of different parental viruses. This process, known as genetic reassortment, is believed to have been the cause of the 1957 and 1968 pandemics.  The 1968 pandemic arose when the H3 hemagglutinin gene and one other internal gene from an avian donor reassorted with the N2 neuraminidase and five other genes from the H2N2 human strain that had been in circulation.
    
   
2. A non-human influenza strain acquires the ability to infect humans.  The 1918 pandemic arose when an avian H1N1 strain mutated to enable its rapid and efficient transfer from human-to-human.
    
   
3. A strain that had previously caused an epidemic may remain sequestered and unaltered within the human population.

Antigenic shift routes 1 & 2 described above. Click the figure to see the enlarged version.

Antigenic shift route 3 described above. Click the figure to see the enlarged version.