We’ve said it before and we will say it again: the Influenza virus is elusive and will forever find ways to outsmart us. Now it seems that the very act of vaccination is the cause of genetic and antigenic evolution.
Outbreaks of Influenza A(H7N3) virus were first detected in Italy in October 2002. The virus continued to circulate between 2002 and 2004 in a densely populated poultry area in the Northeast of the country.
Influenza A(H7N3) virus circulated in unvaccinated and vaccinated poultry farms and the infection was eventually controlled in August 2003 by culling, control of movements, improved bio-security and vaccination.
 showed that viruses isolated in the 2004 outbreaks after the implementation of vaccination acquired specific amino acid signatures, most of which were located at reported antibody binding sites of the haemagglutinin (HA) protein. Antigenic characterization of these 2004 isolates showed that they were antigenically different when compared to those isolated prior to the implementation of vaccination.
This all means that the Influenza A(H7N3) virus has undergone antigenic and genetic evolution following vaccination in poultry. The very act of vaccination resulted in a novel virus.
 Beato et al: Antigenic and genetic evolution of Low Pathogenic Avian influenza viruses (H7N3) following heterologous vaccination in Clinical and Vaccine Immunology - 2014