This image and description give details on how the influenza virus has evolved in clusters. Seasonal vaccinations increase the distance between clusters by selecting against certain strains.
Smith, D.J., A.S. Lapedes, J.C. de Jong, T.M. Bestbroer, G.F. Rimmelswaan, A.D.M.E. Osterhaus et.al. Mapping the Antigenic and Genetic Evolution of Influenza Virus. Science Magazine. 305(5682): 371-376, 2004.
Hannah Millimet