To determine whether DR1-Flu3 multimers could be used to stain CD4+ T cells directly ex vivo, PBMCs were obtained from one healthy DR1+ volunteer before and after immunization with the 2011–2012 trivalent seasonal influenza vaccine, which incorporates the HA306–318 epitope, and in two further DR1+ volunteers that were not vaccinated. This evidence concerns the gene CD4 and influenza.