How often does reassortment of influenza viruses occur?

We know that reassortment occurs frequently in nature. Fortunately, reassortment rarely results in a virus with pandemic potential, though it has done so at least twice in the 20th century.

The influenza viruses that caused the 1957 and 1968 pandemics contained a mixture of gene segments from human and avian influenza viruses.  What is clear from genetic analysis of the viruses that caused these past pandemics is that reassortment (gene swapping) occurred to produce novel influenza viruses that caused the pandemics.  In both of these cases, the new viruses that emerged showed major differences from the parent viruses. 

However, not all viruses emerge directly from reassortment events. For example, the origins of the 1918 virus are not precisely known, but experts think it is likely that the 1918 virus may have resulted from a bird influenza virus directly infecting humans and pigs at about the same time without reassortment.

Last Reviewed: 08/27/2010

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