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Background Takeda’s live attenuated tetravalent dengue vaccine candidate (TAK-003) is under evaluation in a long-term clinical trial across eight dengue-endemic countries. Previously, we have reported its efficacy and safety in both seronegative and seropositive participants and that its performance varies by serotype, with some decline in efficacy from first to second year post-vaccination. This exploratory analysis provides an update with cumulative and third year data. Methods Healthy 4–16 year-olds (n=20,099) were randomized 2:1 to receive TAK-003 or placebo (0, 3 month schedule). The protocol included baseline serostatus testing of all participants and detection of all symptomatic dengue throughout the trial with a serotype specific RT-PCR. Results Cumulative efficacy after three years was 62.0% (95% confidence interval: 56.6%, 66.7%) against virologically-confirmed dengue (VCD) and 83.6% (76.8%, 88.4%) against hospitalized VCD. Efficacy was 54.3% (41.9%, 64.1%) against VCD and 77.1% (58.6%, 87.3%) against hospitalized VCD in baseline seronegatives, and 65.0% (58.9%, 70.1%) against VCD and 86.0% (78.4%, 91.0%) against hospitalized VCD in seropositives. Efficacy against VCD during the third year declined to 44.7% (32.5%, 54.7%), while efficacy against hospitalized VCD was sustained at 70.8% (49.6%, 83.0%). Rates of serious adverse events were 2.9% in TAK-003 group and 3.5% in placebo group during the ongoing long-term follow-up (i.e. second half of the three years following vaccination), but none were related. No important safety risks were identified. Conclusions TAK-003 was efficacious against symptomatic dengue over three years. Efficacy declined over time but remained robust against hospitalized dengue. A booster dose evaluation is planned.

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