Annual influenza vaccination is the most effective strategy available to combat seasonal influenza, but data suggest that repeated vaccination hinders cross-reactive CD8+ T-cell immunity, potentially increasing susceptibility during vaccine mismatch or a pandemic.
In a study carried out in 2010–2013, 25 healthcare workers received a single AS03-adjuvanted H1N1pdm09 vaccine in 2009, and then either repeated annual vaccination or no further vaccination. Blood samples were taken before the influenza season/vaccination to assess antibody and T-cell responses.
In both the repeated- and single-vaccination groups, antibody levels against H1N1pdm09 remained above the protective threshold in most people. CD4+ T cell responses remained stable in the repeated group but declined significantly in the single-vaccination group. Furthermore, the quality of CD4+ responses was higher in the repeated group than the single group. CD8+ T cell responses remained stable in both groups.
Reporting in The Journal of Infectious Diseases, the researchers said the findings suggest that there are immunological benefits from repeated annual vaccination
 Trieu MC, Zhou F, Lartey S et al. Long-term maintenance of the influenza-specific cross-reactive memory CD4+ T-cell responses following repeated annual influenza vaccination. J Infect Dis 2017;215(5):740–749. doi: 10.1093/infdis/jiw619