Influenza virus: a master of metamorphosis.

Novel influenza viruses continuously emerge in the human population. Three times during the present century, an avian influenza virus subtype crossed the species barrier, starting a pandemic, and establishing itself for one to several decades in man. As the 1997 H5N1 event in Hong Kong indicated, the occurrence of another pandemic in the near future cannot be excluded. Sufficient vaccine may not be available to ameliorate the consequences of such an event, because of a shortage of time. During interpandemic periods, important antigenic drift variants sometimes arise at a point of time when, with the current state of the technique, production of a correspondingly adapted vaccine is also impossible. We may be able to solve these problems by increasing influenza surveillance and by adopting new ways of vaccine composition, production, formulation, presentation, and delivery. The recently developed anti-neuraminidase antivirals should only be considered as (valuable) adjuncts to vaccines.