The control of influenza.

Influenza virus differs from the usual agents of epidemic disease by its extreme variability. Since consecutive outbreaks are caused by antigenically different viruses, both herd immunity and vaccination are largely ineffective and the epidemiology is characterized by pandemics, sensu stricto. Severe pandemics occur every 10-12 years, followed by a period (subtype era) over which the evolution of the virus follows a predictably regular course. This process can be imitated in the laboratory, yielding mutant viruses which may serve as prospective vaccines. The transition between subtypes is abrupt and hitherto unpredictable. There are indications, however, that the number of subtypes is limited and that they are linked in a secular cycle of about 70 years. If this proves to be correct, it should be possible to anticipate even the major antigenic shifts and thus eventually fully control the disease.