VDV3, a clonal derivative of the Mahidol live-attenuated dengue 3 vaccine was prepared in Vero cells. Despite satisfactory preclinical evaluation, VDV3 was reactogenic in humans. We explored whether immunological mechanisms contributed to this outcome by monitoring innate and adaptive cellular immune responses for 28 days after vaccination. While no variations were seen in serum IL12 or TNFalpha levels, a high IFNgamma secretion was detected from Day 8, concomitant to IFNalpha, followed by IL10. Specific Th1 and CD8 responses were detected on Day 28, with high IFNgamma/TNFalpha ratios. Vaccinees exhibited very homogeneous class I HLA profiles, and a new HLA B60-restricted CD8 epitope was identified in NS3. We propose that, among other factors, adaptive immunity may have contributed to reactogenicity, even after this primary vaccination. In addition, the unexpected discordance observed between preclinical results and clinical outcome in humans led us to reconsider some of our preclinical acceptance criteria. Lessons learned from these results will help us to pursue the development of safe and immunogenic vaccines.