Study Population

The case cohort included the 3578 participants in the subcohort (2384 in the vaccine group and 1194 in control group), as well as all the participants from the trials who had symptomatic VCD (1258 cases), hospitalization for VCD (644 cases), or severe VCD (142 cases) (Table S2 in the Supplementary Appendix). The distribution of baseline demographic characteristics in the overall, seropositive, and seronegative populations in the subcohort was balanced between the vaccine group and the control group; 24.5% of the participants were classified as seronegative by logistic regression (multiple-imputation), 24.0% by super learner (targeted minimum loss–based estimation), and 23.4% by measurement of anti-NS1 titers (with the cutoff of 9 EUs per milliliter). In the subcohort, data on month 0 PRNT 50 and month 13 anti-NS1 titers were missing for 66.7% (1591 of 2384) and 3.9% (93 of 2384), respectively, of vaccine recipients and 68.5% (818 of 1194) and 4.8% (57 of 1194) of controls. If (as is plausible when this sampling design is used) the probabilities of missing data are random after measured variables used in the analysis have been accounted for, then the inferences are expected to be valid. Additional details on the baseline characteristics of the participants and on missing data are provided in Tables S1, S3 through S6, and S46 in the Supplementary Appendix.

Validation of Imputed Baseline Dengue Serostatus

The accuracy of the logistic-regression and super learner models was cross-validated for the predictability of measured baseline PRNT 50 serostatus; the methods produced similar results, with 79% of the participants who had been predicted to be seronegative by each method confirmed to be seronegative on the basis of measured PRNT 50 titers (Table S7 in the Supplementary Appendix). The analysis of concordance between the anti-NS1 titers at month 13 and PRNT 50 titers at month 0 and the analysis of the effect of CYD-TDV on the anti-NS1 titers at month 13 are provided in Table S8 and pages 19 through 23, respectively, in the Supplementary Appendix.

Safety and Efficacy Estimates

Because the estimates calculated by the different analytic approaches were generally consistent with one another, pooled estimates based on the multiple-imputation approach (which is more commonly used for handling issues with missing data) from month 0 onward are reported unless indicated otherwise; estimates from all the methods we used are provided in the tables, figures, and Supplementary Appendix. Exploratory analyses showed statistical evidence of interaction between serostatus and treatment effect for the safety and efficacy end points (P<0.01 for all comparisons) and support the separate presentation of estimates for seronegative and seropositive populations. No statistical evidence of interaction between age groups (2 to 8 years and 9 to 16 years of age) and treatment effect on the safety end points in seronegative participants was observed in exploratory analyses, and therefore P values for these end points are reported only for the analyses involving participants 2 to 16 years of age.

Risk Associated with Vaccination at 9 to 16 Years of Age

Figure 1. Figure 1. Risk of Hospitalization for Virologically Confirmed Dengue (VCD) and of Severe VCD in Participants 9 to 16 Years of Age, According to Baseline Serostatus. Dengue serostatus was assigned on the basis of multiple imputation (MI, month 0 onward), targeted minimum loss–based estimation (TMLE, month 0 onward), and measured anti–nonstructural protein 1 (NS1) titer (threshold for positivity, 9 enzyme-linked immunosorbent assay [ELISA] units per milliliter [T9]; month 13 onward). Hazard ratios (MI and NS1) and relative risks (TMLE) are shown with corresponding 95% confidence intervals. For NS1, numerators represent the number of participants who were hospitalized for VCD or had severe VCD, and the denominators are the total numbers of participants selected in the subcohort. For MI, the numerators and denominators are the means of 10 iterations of MI, with the numerator representing the number of participants who were hospitalized for VCD or had severe VCD and the denominator representing the total number of participants selected in the subcohort. For TMLE, the numerators are the predicted numbers of study group–specific events among participants of the given serostatus within the subcohort, and the denominators are the predicted numbers of participants of a given serostatus within the subcohort. The analysis involved the as-treated population, in which participants were classified as being in the vaccine group if they had received at least one injection of CYD-TDV. Data were pooled from the CYD14, CYD15, and CYD23 (and CYD57) trials.

Among seronegative participants 9 to 16 years of age, the hazard ratio (vaccine vs. control) for hospitalization for VCD was 1.41 (95% confidence interval [CI], 0.74 to 2.68) and that for severe VCD was 2.44 (95% CI, 0.47 to 12.56); the point estimates of the hazard ratio and relative risk were greater than 1 for all methods in the pooled analyses (Figure 1, and Table S9 in the Supplementary Appendix). Through month 60 among seronegative participants, the cumulative incidence of hospitalization for VCD was 1.57% (95% CI, 1.13 to 2.19) in the vaccine group and 1.09% (95% CI, 0.53 to 2.27) in the control group, and the incidence of severe VCD was 0.40% (95% CI, 0.22 to 0.75) in the vaccine group and 0.17% (95% CI, 0.04 to 0.83) in the control group. Additional details of the results for seronegative participants, as well as exploratory analyses for each trial, are provided in Tables S10 through S12 in the Supplementary Appendix.

Among seropositive participants, the hazard ratio (vaccine vs. control) for hospitalization for VCD was 0.21 (95% CI, 0.14 to 0.31) and that for severe VCD was 0.16 (95% CI, 0.07 to 0.37); the point estimates of the hazard ratios and relative risk were less than 1 with all methods in the pooled analyses (Figure 1, and Table S9 in the Supplementary Appendix) and in the individual trials (Table S13 in the Supplementary Appendix). Table S14 in the Supplementary Appendix shows exploratory analyses involving seropositive participants who were 9 to 11 years and 12 to 16 years of age. Through month 60 among seropositive participants, the cumulative incidence of hospitalization for VCD was 0.38% (95% CI, 0.26 to 0.54) in the vaccine group and 1.88% (95% CI, 1.54 to 2.31) in the control group, and the cumulative incidence of severe VCD was 0.08% (95% CI, 0.03 to 0.17) in the vaccine group and 0.48% (95% CI, 0.34 to 0.69) in the control group (Table S10 in the Supplementary Appendix).

The attributable risk over a 60-month period per 1000 seronegative vaccine recipients was 4.78 (95% CI, −13.99 to 24.00) for hospitalization for VCD and 2.30 (95% CI, −7.00 to 10.67) for severe VCD. The corresponding attributable risk per 1000 seropositive vaccine recipients was −15.08 (95% CI, −25.44 to −4.97) and −4.05 (95% CI, −9.59 to 0.63), respectively.

Risk Associated with Vaccination at 2 to 8 Years of Age

Figure 2. Figure 2. Risk of Hospitalization for VCD and of Severe VCD in Participants 2 to 8 Years and 2 to 16 Years of Age, According to Baseline Serostatus. Dengue serostatus was categorized on the basis of MI (month 0 onward), TMLE (month 0 onward), and NS1 (threshold for positivity, 9 ELISA units per milliliter [T9]; month 13 onward). Hazard ratios (MI and NS1) and relative risks (TMLE) are shown with corresponding 95% confidence intervals. For NS1, numerators represent the number of participants who were hospitalized for VCD or had severe VCD, and the denominators are the total numbers of participants selected in the subcohort. For MI, the numerators and denominators are means from 10 iterations of MI, with the numerator representing the number of participants who were hospitalized for VCD or had severe VCD and the denominator representing the total number of participants selected in the subcohort. For TMLE, the numerators are the predicted numbers of study group–specific events among participants of the given serostatus within the subcohort, and the denominators are the predicted numbers of participants of a given serostatus within the subcohort. The analysis involved the as-treated population, in which participants were classified as being in the vaccine group if they had received at least one injection of CYD-TDV. Data were pooled from the CYD14, CYD15, and CYD23 (and CYD57) trials. For the analysis involving participants who were 2 to 16 years of age, the unadjusted and adjusted P values for the comparisons between the vaccine group and the control group among seronegative participants were as follows: MI method for hospitalization for VCD, P=0.01 (unadjusted) and P=0.02 (Holm–Bonferroni adjusted); MI method for severe VCD, P=0.03 (unadjusted) and P=0.03 (Holm–Bonferroni adjusted); TMLE method for hospitalization for VCD, P=0.07 (unadjusted) and P=0.09 (Holm–Bonferroni adjusted); and TMLE method for severe VCD, P=0.04 (unadjusted) and P=0.09 (Holm–Bonferroni adjusted). Holm–Bonferroni adjustment was performed for the two safety end points (two tests) independently for each method.

Among seronegative participants 2 to 8 years of age, the hazard ratio (vaccine vs. control) for hospitalization for VCD was 1.95 (95% CI, 1.19 to 3.19) and that for severe VCD was 3.31 (95% CI, 0.87 to 12.54) (Figure 2A); the point estimates were greater than 1 for all methods in pooled analyses and in individual trials. Among seropositive participants, the corresponding hazard ratios were 0.50 (95% CI, 0.33 to 0.77) and 0.58 (95% CI, 0.26 to 1.30) (Figure 2A), and the point estimates were less than 1 in all analyses. Additional results, including the estimates of cumulative incidence and attributable risk and the results of exploratory analyses involving seronegative participants who were 2 to 5 years and 6 to 8 years of age, are shown in Tables S11 and S14 through S18 in the Supplementary Appendix).

Risk Associated with Vaccination at 2 to 16 Years of Age

Among seronegative participants who were 2 to 16 years of age, the hazard ratio (vaccine vs. control) for hospitalization for VCD was 1.75 (95% CI, 1.14 to 2.70) and that for severe VCD was 2.87 (95% CI, 1.09 to 7.61) (Figure 2B); all point estimates were greater than 1. Among seronegative participants, the cumulative incidence of hospitalization for VCD through month 60 was 3.06% (95% CI, 2.53 to 3.61) among vaccine recipients and 1.87% (95% CI, 1.23 to 2.86) among controls. Among seropositive participants, the corresponding hazard ratios were 0.32 (95% CI, 0.23 to 0.45) and 0.31 (95% CI, 0.17 to 0.58) (Figure 2B), and point estimates were less than 1 in all analyses. Among seropositive participants, the cumulative incidence of hospitalization for VCD through month 60 was 0.75% (95% CI, 0.56 to 1.00) among vaccine recipients and 2.47% (95% CI, 2.09 to 2.92) among controls. Additional results are provided in Tables S19 through S22 in the Supplementary Appendix.

Risk over Time

Figure 3. Figure 3. Cumulative Incidence Curves of Hospitalization for VCD from Month 0 According to Baseline Serostatus as Classified by PRNT 50 at Baseline (Multiple-Imputation Approach) in Different Age Groups. Data are from a pooled analysis of the CYD14, CYD15, and CYD23 (and CYD57) trials. The cumulative incidence curves are curtailed at month 66 to ensure that at least 20% of the participants remained at risk in each subcohort. Insets show the same data on an enlarged y axis.

Among seronegative participants, the hazard ratio (vaccine vs. control) for hospitalization for VCD was greater than 1 as estimated with most methods during the hospital phase (month 25 onward) in participants who were 9 to 16 years of age (Table S23 in the Supplementary Appendix). The risk estimates according to time period among participants who were 2 to 8 years or 2 to 16 years of age are shown in Tables S24 and S25 in the Supplementary Appendix. There was an excess risk of hospitalization for VCD in seronegative vaccine recipients as compared with seronegative controls from month 30 onward among those who were 9 to 16 years of age and from month 18 onward among those who were 2 to 8 years of age (Figure 3). Among seropositive participants, the cumulative risk was lower in the vaccine group than in the control group throughout follow-up (Figure 3, and Figs. S1 and S2 in the Supplementary Appendix).

Clinical Profile of Cases and Risk According to Dengue Virus Serotype

Table 1. Table 1. Clinical Signs and Symptoms in All Hospitalizations for Virologically Confirmed Dengue (VCD) Occurring from Month 13 to the End of the Follow-up Period (Month 60 to Month 72) among Seronegative Participants.

In all age groups, the median duration of fever, symptoms, and hospitalization did not differ between hospitalized patients with VCD in the vaccine group and those in the control group. A higher risk of plasma leakage and severe thrombocytopenia (platelet count, <50×109 per liter) was found in the vaccine group (Table 1). The overall clinical picture among patients with severe VCD was similar in the two study groups, and most cases were dengue hemorrhagic fever (DHF) grade I or II, as defined in the World Health Organization (WHO) 1997 classification (Tables S26 and S27 in the Supplementary Appendix). All the affected participants recovered. There were no dengue-related deaths. The all-cause mortality rate in all the trials combined was 0.24% (0.21% in the vaccine group and 0.30% in the control group), and no deaths from any cause were judged by the investigators and the sponsor to be related to the vaccine. No differences in the symptomatology between cases in seronegative vaccine recipients and those in seropositive controls were found (Tables S28 through S31 in the Supplementary Appendix). Among seronegative participants, the hazard ratio or relative risk (vaccine vs. control) of hospitalization for VCD caused by serotype 1 or 3 dengue virus was greater than 1 in some analyses, and these ratios were consistently greater than 1 for hospitalization for VCD caused by serotype 2 dengue virus. Among seropositive participants, the hazard ratios and relative risks of hospitalization for VCD were less than 1 for dengue due to each of the four serotypes (Table S32 and S33 in the Supplementary Appendix).

Vaccine Efficacy against Symptomatic VCD

Figure 4. Figure 4. Vaccine Efficacy against Symptomatic VCD up to Month 25 According to Baseline Serostatus in Different Age Groups. Dengue serostatus was categorized on the basis of MI (month 0 onward), TMLE (month 0 onward), and NS1 (threshold for positivity, 9 ELISA units per milliliter [T9]; month 13 onward). Vaccine efficacy estimates are shown with corresponding 95% confidence intervals. For NS1, the numerators represent the number of participants who had symptomatic VCD and the denominators represent the total participants selected in the subcohort; estimates are from month 13 to month 25. For MI, the numerators and denominators are means from 10 iterations of MI, with the numerator representing the number of participants with symptomatic VCD and the denominator representing the total number of participants selected in the subcohort; estimates are from month 0 to month 25. For TMLE, the numerators are the predicted numbers of study group–specific events among participants of the given serostatus within the subcohort and the denominators are the predicted numbers of participants of a given serostatus within the subcohort; estimates are from month 0 to month 25. The analysis involved the intention-to-treat population, with participants included in the group (vaccine or control) to which they had been randomly assigned. Data were pooled from the CYD14 and CYD15 trials.

Among seronegative participants, vaccine efficacy against symptomatic VCD (up to month 25) was 39% (95% CI, −1 to 63) among those who were 9 to 16 years of age, 19% (95% CI, −47 to 55) among those who were 2 to 8 years of age, and 32% (95% CI, −9 to 58) among those who were 2 to 16 years of age. Among seropositive participants, the corresponding values were 76% (95% CI, 64 to 84), 60% (95% CI, 31 to 76), and 73% (95% CI, 59 to 82) (Figure 4). Vaccine efficacy as determined by all other methods and according to trial, serotype, and age stratum are shown in Tables S34 through S43 in the Supplementary Appendix.