Vaccine-specific and bystander proliferative responses of T_mem cells in 12 individuals following booster vaccination with TT. Responses are shown at the week of maximal TT response and are compared with the prevaccine (week 0) response. Proliferative activity was measured by ³H-thymidine incorporation after 6 days of in vitro culture and is reported as the mean value of stimulation index (SI) in triplicate cultures ± SD.