New criteria for the selection of influenza vaccine strains.

Antigenic relationships are defined by 2 criteria: distance and symmetry. Viruses within a subtype of influenza A are closely related, with the more junior antigens being superseded in time by senior ones. This phenomenon can be reproduced experimentally. Subtypes are distantly related to one another.Immunity to a junior virus is not readily transformed into immunity to a senior antigen given later (original antigenic sin), whereas senior viruses do not jeopardize the efficiency of junior vaccines given subsequently.It is proposed that future vaccines should contain senior mutants of a strain from the most recent epidemic (prospective vaccines), and no junior antigen.