Background: We are conducting a multicenter randomized phase II trial of AE37, the Ii-Key hybrid peptide of HER2 776-790 (AE36). The purpose of the study is to determine if the AE37 vaccine can prevent recurrence in disease-free conventionally treated node-positive (NP) and high-risk node-negative (NN) breast cancer patients at significant risk for recurrence. Since clinical efficacy is anticipated to occur as the result of long lasting memory immune responses induced by vaccination, repeated booster inoculations were scheduled as part of the trial. Here we present data on immune responses in patients who received boosters up to 24 months after completion of the primary vaccination series (PVS). Methods: The trial is enrolling NP or high-risk NN patients with any degree of HER2 expression (IHC 1-3+ or FISH > 1.2) rendered disease-free following standard of care therapy. The vaccine group (VG) received AE37+GM-CSF and control group (CG) GM-CSF alone in 6 monthly i.d. inoculations followed by boosters administered every 6 months x 4. Immunologic responses were assessed in vivo by dermal reactions at the inoculation site, and in vitro, against the AE36 peptide, with proliferation and IFN-γ ELISPOT assays. Results: 25 patients in the VG and 23 in the CG have completed their boosters. After the last booster (BRC24), 100%, 54% and 54% in the VG (vs. 9%, 18% and 27% in the CG) responded by dermal reaction, proliferation and IFN-γ ELISPOT, respectively. Mean dermal reactions (orthogonal mean in mm) in vaccinated patients was 25.9±3.13 at completion of the PVS (R6) and increased to 35.47±4.35 at BRC24 (p=0.01). VG patients increased their proliferation response (stimulation index, SI) to AE36 from 0.97±0.046 at baseline (R0) before vaccination to 2.27±0.57 at R6 (p=0.0003) which was maintained until BRC24 (SI 2.21±0,33, p<0.0001). The number of IFN-γ specific spots/10⁶ PBMC increased from 26.88±12.36 at R0 to 40.35±17.02 (p=0.07) at R6, up to 62±16.82 (p=0.0076) at BRC24. Conclusions: Our data demonstrate that AE37 vaccine boosters enhance the immune responses against HER elicited during the PVS, thus sustaining long lasting immunity, a prerequisite for possible clinical efficacy which is currently being evaluated. Clinical trial information: NCT00524277.