264 T. FRANCIS, JR. 



Subgroups showing less distinct variation from each other occur within the 

 major groups and represent random deviations about initial prototype strains 

 of the group (Jensen and Peterson, 1957). This variation appears more as an 

 immunological drift (Burnet, 1953) in the relative expression of their multiple 

 common antigens rather than a directional shift. 



TABLE III 

 Influenza Vietjs, Type A 



Group Prototype Common term Prevalence 



The differences between groups A 2 , A 3 , and A 4 and the similarity of strains 

 within the groups is further illustrated by the effects of vaccination. Vaccine 

 prepared from one of several strains may protect against epidemics related to 

 strains of different subgroups, but strains from one group have not been 

 highly effective in protection against disease caused by strains of another. 

 Nevertheless, evidence indicates that polyvalent vaccine of certain groups 

 can provide a significant degree of protection against strains of a heterologous 

 group (Commission on Influenza, 1957). And it seems likely that the essential 

 antigens representative of all type A strains may be accumulated into one 

 vaccine which will provide a composite immunity to the entire type. 



The extensive strain relationships and the limited number of major 

 variants appear in keeping with the hypothesis that a finite number of antigens 

 is represented in type A virus and that variation is primarily rearrangement 

 of the constituents. Moreover, if antibodies and immunity of the population 

 are significant factors in the determination of the variants which arise, 

 recurrent or cyclic resurgences of an antigen can be expected. The young 

 segment of the population is always the most susceptible and it has been 

 increasingly deficient in antibodies to the swine group, the PR8 group, and, 

 until recently, to the Asian group of strains. Immune pressure might then 

 direct variation into this wide gap where numerous alterations could be 

 effectively distributed. The Asian variant of 1957 seems to have been so 

 derived. There remains, however, the possibility that the major variants 

 are recurrences of fixed strains which have remained antigenically intact in a 

 reservoir or by scanty distribution, then to regain epidemic status when 

 opportunity permits. The return of virulence would be more an influence of 

 the host population than of antigenic variation as currently recognized. The 



