Bayesian borrowing from historical control data in a vaccine efficacy trial.

In the context of vaccine efficacy trial where the incidence rate is very low and a very large sample size is usually expected, incorporating historical data into a new trial is extremely attractive to reduce sample size and increase estimation precision. Nevertheless, for some infectious diseases, seasonal change in incidence rates poses a huge challenge in borrowing historical data and a critical question is how to properly take advantage of historical data borrowing with acceptable tolerance to between-trials heterogeneity commonly from seasonal disease transmission. In this article, we extend a probability-based power prior which determines the amount of information to be borrowed based on the agreement between the historical and current data, to make it applicable for either a single or multiple historical trials available, with constraint on the amount of historical information to be borrowed. Simulations are conducted to compare the performance of the proposed method with other methods including modified power prior (MPP), meta-analytic-predictive (MAP) prior and the commensurate prior methods. Furthermore, we illustrate the application of the proposed method for trial design in a practical setting.

Analysis of Neutralizing Antibodies as a Correlate of Instantaneous Risk of Hospitalized Dengue in Placebo Recipients of Dengue Vaccine Efficacy Trials.

BACKGROUND: In the CYD14 (NCT01373281) and CYD15 (NCT01374516) dengue vaccine efficacy trials, month 13 neutralizing antibody (nAb) titers correlated inversely with risk of symptomatic, virologically confirmed dengue (VCD) between month 13 (1 month after final dose) and month 25. We assessed nAb titer as a correlate of instantaneous risk of hospitalized VCD (HVCD), for which participants were continually surveilled for 72 months. METHODS: Using longitudinal nAb titers from the per-protocol immunogenicity subsets, we estimated hazard ratios (HRs) of HVCD by current nAb titer value for 3 correlate/endpoint pairs: average titer across all 4 serotypes/HVCD of any serotype (HVCD-Any), serotype-specific titer/homologous HVCD, and serotype-specific titer/heterologous HVCD. RESULTS: Baseline-seropositive placebo recipients with higher average titer had lower instantaneous risk of HVCD-Any in 2- to 16-year-olds and in 9- to 16-year-olds (HR, 0.26 or 0.15 per 10-fold increase in average titer by 2 methods [95% confidence interval {CI}, .14-.45 and .07-.34, respectively]) pooled across both trials. Results were similar for homologous HVCD. There was evidence suggesting increased HVCD-Any risk in participants with low average titer (1:10 to 1:100) compared to seronegative participants (HR, 1.85 [95% CI, .93-3.68]). CONCLUSIONS: Natural infection-induced nAbs were inversely associated with hospitalized dengue, upon exceeding a relatively low threshold.

Immunobridging efficacy of a tetravalent dengue vaccine against dengue and against hospitalized dengue from children/adolescents to adults in highly endemic countries.

BACKGROUND: The recombinant tetravalent live-attenuated dengue vaccine based on the YF 17D vaccine virus backbone (CYD-TDV) demonstrated vaccine efficacy (VE) against symptomatic, virologically confirmed dengue of any serotype from month 13 to month 25 (VCD-DENV-AnyM13→M25) in the CYD14 (2-14-y-olds) and CYD15 (9-16-y-olds) phase 3 trials. Fifty percent plaque reduction neutralization test (PRNT50) titers are a potential surrogate for immunobridging VE to adults. METHODS: Using PRNT50 calibration datasets, we applied immunobridging approaches using baseline and/or M13 PRNT50 titers to estimate VE against VCD-DENV-AnyM0→M25 and against hospitalized VCD (HVCD)-DENV-AnyM0→M72 in hypothetical 18-45-y-old and 46-50-y-old CYD14 and CYD15 cohorts. RESULTS: Baseline and M13 geometric mean PRNT50 titers were greater in 18-45-y-olds and in 46-50-y-olds vs 9-16-y-olds for most comparisons. Estimated VE (95% CIs against VCD-DENV-AnyM0→M25 ranged from 75.3% to 90.9% (52.5% to 100%) for 18-45-y-olds and 74.8% to 92.0% (53.4% to 100%) for 46-50-y-olds. Estimated VE (95% CIs) against HVCD-DENV-AnyM0→M72 ranged from 58.8% to 78.1% (40.9 to 98.9%) for 18-45-y-olds and 57.2% to 78.4% (40.5 to 97.6%) for 46-50-y-olds. Corresponding predictions among baseline-seropositive individuals yielded comparable or higher VE estimates. CONCLUSIONS: VE M0→M25 against DENV-Any and VE against HVCD-DENV-AnyM0→M72 are both expected to be higher in 18-45 and 46-50-y-olds vs CYD14 and CYD15 9-16-y-olds.

Microneutralization assay titer correlates analysis in two phase 3 trials of the CYD-TDV tetravalent dengue vaccine in Asia and Latin America.

We previously showed that Month 13 50% plaque reduction neutralization test (PRNT50) neutralizing antibody (nAb) titers against dengue virus (DENV) correlated with vaccine efficacy (VE) of CYD-TDV against symptomatic, virologically-confirmed dengue (VCD) in the CYD14 and CYD15 Phase 3 trials. While PRNT is the gold standard nAb assay, it is time-consuming and costly. We developed a next-generation high-throughput microneutralization (MN) assay and assessed its suitability for immune-correlates analyses and immuno-bridging applications. We analyzed MN and PRNT50 titers measured at baseline and Month 13 in a randomly sampled immunogenicity subset, and at Month 13 in nearly all VCD cases through Month 25. For each serotype, MN and PRNT50 titers showed high correlations, at both baseline and Month 13, with MN yielding a higher frequency of baseline-seronegatives. For both assays, Month 13 titer correlated inversely with VCD risk. Like PRNT50, high Month 13 MN titers were associated with high VE, and estimated VE increased with average Month 13 MN titer. We also studied each assay as a valid surrogate endpoint based on the Prentice criteria, which supported each assay as a valid surrogate for DENV-1 but only partially valid for DENV-2, -3, and -4. In addition, we applied Super-Learner to assess how well demographic, Month 13 MN, and/or Month 13 PRNT50 titers could predict Month 13-25 VCD outcome status; prediction was best when using demographic, MN, and PRNT50 information. We conclude that Month 13 MN titer performs comparably to Month 13 PRNT50 titer as a correlate of risk, correlate of vaccine efficacy, and surrogate endpoint. The MN assay could potentially be used to assess nAb titers in immunogenicity studies, immune-correlates studies, and immuno-bridging applications. Additional research would be needed for assessing the utility of MN titer in correlates analyses of other DENV endpoints and over longer follow-up periods.

Bridging Efficacy of a Tetravalent Dengue Vaccine from Children/Adolescents to Adults in Highly Endemic Countries Based on Neutralizing Antibody Response.

The CYD-TDV vaccine is licensed in multiple endemic countries based on vaccine efficacy (VE) against symptomatic, virologically confirmed dengue demonstrated in two phase 3 trials (CYD14, 2- to 14-year-olds, Asia; CYD15, 9- to 16-year-olds, Latin America). 50% plaque reduction neutralization test (PRNT50) titers at baseline and month 13 (post-vaccination) were associated with VE and may enable bridging VE to adults. Two phase 2 trials of CYD-TDV measured baseline and month 13 PRNT50 titers: CYD22 (9- to 45-year-olds, Vietnam) and CYD47 (18- to 45-year-olds, India). 50% plaque reduction neutralization test distributions were compared between age cohorts, and four versions of an epidemiological bridging method were used to estimate VE against any serotype (dengue virus [DENV]-Any) and against each serotype over 25 months post first vaccination in a hypothetical CYD14 + CYD15 18- to 45-year-old cohort (bridging population 1) and in the actual CYD47 18- to 45-year-old cohort (bridging population 2). Baseline and month 13 geometric mean PRNT50 titers to each serotype were significantly greater in 18- to 45-year-olds than 9- to 16-year-olds for all comparisons. The four methods estimated VE against DENV-Any at 75.3-86.0% (95% CIs spanning 52.5-100%) for bridging population 1 and 68.4-77.5% (95% CIs spanning 42.3-88.5%) for bridging population 2. The vaccine efficacy against serotype 1, 2, 3, and 4 was estimated at 56.9-76.9%, 68.3-85.8%, 91.4-95.0%, and 93.2-100% (bridging population 1) and 44.5-66.9%, 53.2-69.2%, 79.8-92.0%, and 90.6-95.0% (bridging population 2), respectively; thus, CYD-TDV would likely confer improved efficacy in adults than 9- to 16-year-olds. Using the same methods, we predicted VE against hospitalized DENV-Any over 72 months of follow-up, with estimates 59.1-73.5% (95% CIs spanning 40.9-92.2%) for bridging population 1 and 50.9-65.9% (95% CIs spanning 38.1-82.1%) for bridging population 2.

Periods of high dengue transmission defined by rainfall do not impact efficacy of dengue vaccine in regions of endemic disease.

OBJECTIVE: To evaluate the association of rainy season with overall dengue disease incidence and with the efficacy of the Sanofi Pasteur recombinant, live, attenuated, tetravalent vaccine (CYD-TDV) in two randomized, controlled multicenter phase III clinical trials in Asia and Latin America. METHODS: Rainy seasons were defined for each study site using climatological information from the World Meteorological Organization. The dengue attack rate in the placebo group for each study month was calculated as the number of symptomatic, virologically-confirmed dengue events in a given month divided by the number of participants at risk in the same month. Time-dependent Cox proportional hazard models were used to test whether rainy season was associated with dengue disease and whether it modified vaccine efficacy in each of the two trials and in both of the trials combined. FINDINGS: Rainy season, country, and age were all significantly associated with dengue disease in both studies. Vaccine efficacy did not change during the rainy season in any of the analyses. CONCLUSIONS: Although dengue transmission and exposure are expected to increase during the rainy season, our results indicate that CYD-TDV vaccine efficacy remains constant throughout the year in endemic regions.

Neutralizing Antibody Correlates Analysis of Tetravalent Dengue Vaccine Efficacy Trials in Asia and Latin America.

Background: In the CYD14 and CYD15 Phase 3 trials of the CYD-TDV dengue vaccine, estimated vaccine efficacy (VE) against symptomatic, virologically confirmed dengue (VCD) occurring between months 13 and 25 was 56.5% and 60.8%, respectively. Methods: Neutralizing antibody titers to the 4 dengue serotypes in the CYD-TDV vaccine insert were measured at month 13 in a randomly sampled immunogenicity subcohort and in all VCD cases through month 25 (2848 vaccine, 1574 placebo) and studied for their association with VCD and with the level of VE to prevent VCD. Results: For each trial and serotype, vaccinees with higher month 13 titer to the serotype had significantly lower risk of VCD with that serotype (hazard ratios, 0.19-0.43 per 10-fold increase). Moreover, for each trial, vaccinees with higher month 13 average titer to the 4 serotypes had significantly higher VE against VCD of any serotype (P < .001). Conclusions: Neutralizing antibody titers postdose 3 correlate with CYD-TDV VE to prevent dengue. High titers associate with high VE for all serotypes, baseline serostatus groups, age groups, and both trials. However, lowest titers do not fully correspond to zero VE, indicating that other factors influence VE.

Tetravalent Dengue Vaccine Reduces Symptomatic and Asymptomatic Dengue Virus Infections in Healthy Children and Adolescents Aged 2-16 Years in Asia and Latin America.

BACKGROUND: Asymptomatic dengue virus-infected individuals are thought to play a major role in dengue virus transmission. The efficacy of the recently approved quadrivalent CYD-TDV dengue vaccine against asymptomatic dengue virus infection has not been previously assessed. METHODS: We pooled data for 3 736 individuals who received either CYD-TDV or placebo at 0, 6, and 12 months in the immunogenicity subsets of 2 phase 3 trials (clinical trials registration NCT01373281 and NCT01374516). We defined a seroconversion algorithm (ie, a ≥4-fold increase in the neutralizing antibody titer and a titer of ≥40 from month 13 to month 25) as a surrogate marker of asymptomatic infection in the vaccine and placebo groups. RESULTS: The algorithm detected seroconversion in 94% of individuals with a diagnosis of virologically confirmed dengue between months 13 and 25, validating its discriminatory power. Among those without virologically confirmed dengue (n = 3 669), 219 of 2 485 in the vaccine group and 157 of 1 184 in the placebo group seroconverted between months 13 and 25, giving a vaccine efficacy of 33.5% (95% confidence interval [CI], 17.9%-46.1%) against asymptomatic infection. Vaccine efficacy was marginally higher in subjects aged 9-16 years (38.6%; 95% CI, 22.1%-51.5%). The annual incidence of asymptomatic dengue virus infection in this age group was 14.8%, which was 4.4 times higher than the incidence for symptomatic dengue (3.4%). CONCLUSIONS: The observed vaccine efficacy against asymptomatic dengue virus infections is expected to translate into reduced dengue virus transmission if sufficient individuals are vaccinated in dengue-endemic areas.

Lot-to-lot consistency of a tetravalent dengue vaccine in healthy adults in Australia: a randomised study.

BACKGROUND: The recombinant yellow fever-17D-dengue virus, live, attenuated, tetravalent dengue vaccine (CYD-TDV) has undergone extensive clinical trials. Here safety and consistency of immunogenicity of phase III manufacturing lots of CYD-TDV were evaluated and compared with a phase II lot and placebo in a dengue-naïve population. METHODS: Healthy 18-60 year-olds were randomly assigned in a 3:3:3:3:1 ratio to receive three subcutaneous doses of either CYD-TDV from any one of three phase III lots or a phase II lot, or placebo, respectively in a 0, 6, 12 month dosing schedule. Neutralising antibody geometric mean titres (PRNT50 GMTs) for each of the four dengue serotypes were compared in sera collected 28 days after the third vaccination-equivalence among lots was demonstrated if the lower and upper limits of the two-sided 95% CIs of the GMT ratio were ≥0.5 and ≤2.0, respectively. RESULTS: 712 participants received vaccine or placebo and 614 (86%) completed the study; 17 (2.4%) participants withdrew after adverse events. Equivalence of phase III lots was demonstrated for 11 of 12 pairwise comparisons. One of three comparisons for serotype 2 was not statistically equivalent. GMTs for serotype 2 in phase III lots were close to each other (65.9, 44.1 and 58.1, respectively). CONCLUSIONS: Phase III lots can be produced in a consistent manner with predictable immune response and acceptable safety profile similar to previously characterised phase II lots. The phase III lots may be considered as not clinically different as statistical equivalence was shown for serotypes 1, 3 and 4 across the phase III lots. For serotype 2, although equivalence was not shown between two lots, the GMTs observed in the phase III lots were consistently higher than those for the phase II lot. As such, in our view, biological equivalence for all serotypes was demonstrated.

Cell-mediated immunity induced by chimeric tetravalent dengue vaccine in naive or flavivirus-primed subjects.

Three independent, phase 1 clinical trials were conducted in Australia and in USA to assess the safety and immunogenicity of sanofi pasteur dengue vaccine candidates. In this context, Dengue 1-4 and Yellow Fever 17D-204 (YF 17D)-specific CD4 and CD8 cellular responses induced by tetravalent chimeric dengue vaccines (CYD) were analyzed in flavivirus-naive or flavivirus-immune patients. Tetravalent CYD vaccine did not trigger detectable changes in serum pro-inflammatory cytokines, whatever the vaccinees immune status, while inducing significant YF 17D NS3-specific CD8 responses and dengue serotype-specific T helper responses. These responses were dominated by serotype 4 in naive individuals, but a booster vaccination (dose #2) performed 4 months following dose #1 broadened serotype-specific responses. A similar, broader response was seen after primary tetravalent immunization in subjects with pre-existing dengue 1 or 2 immunity caused by prior monovalent live-attenuated dengue vaccination. In all three trials, the profile of induced response was similar, whatever the subjects' immune status, i.e. an absence of Th2 response, and an IFN-gamma/TNF-alpha ratio dominated by IFN-gamma, for both CD4 and CD8 responses. Our results also showed an absence of cross-reactivity between YF 17D or Dengue NS3-specific CD8 responses, and allowed the identification of 3 new CD8 epitopes in the YF 17D NS3 antigen. These data are consistent with the previously demonstrated excellent safety of these dengue vaccines in flavivirus-naive and primed individuals.

Age-specific prevalence of dengue antibodies in Bangkok infants and children.

Seroprevalence of dengue antibodies were determined in healthy children in Bangkok. At 9, 12, and 18 months of age 23%, 9%, and 17% of children respectively had neutralizing antibodies against one or more serotypes. DEN1 was the most prevalent, followed by DEN3, DEN2, and DEN4. Twelve children had serologic evidence of dengue infection. The nadir of dengue antibodies in children was at 12 months of age. In highly endemic areas, dengue vaccination could be needed at an early age.

Dengue virus infections in the first 2 years of life and the kinetics of transplacentally transferred dengue neutralizing antibodies in thai children.

BACKGROUND: Understanding dengue virus infection in children and the kinetics of maternal dengue neutralizing antibodies is essential for effective dengue immunization of children in endemic areas. METHODS: Serum samples from 219 mother-child pairs and 140 children at 3, 6, 9, 12, 18, and 24 months of age from Bangkok, Thailand, were tested for serotype-specific dengue antibodies. Febrile episodes in the children were recorded. RESULTS: Antibodies were found in 97% of cord serum samples and disappeared in 27%, 80%, and 95% of the children by the age of 6, 9, and 12 months, respectively. Geometric mean titers (GMTs) of the antibodies to 4 dengue serotypes decreased to 5.4-15.5 in 6-month-old infants. Eleven of 12 children acquired dengue virus infection at 6 months of age and beyond; 1 had the infection at 3 months of age. Two exhibited undifferentiated febrile illnesses, and 10 had subclinical infections. CONCLUSIONS: Evidence of dengue virus infection and very low GMTs against all dengue serotypes in children at 6 months of age and beyond was demonstrated. There was no evidence that maternal antibodies were harmful to infants. Dengue virus infection rates increase from 12 months of age onward. These data provide information for supporting the optimal age at vaccination.

Innate and adaptive cellular immunity in flavivirus-naïve human recipients of a live-attenuated dengue serotype 3 vaccine produced in Vero cells (VDV3).

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.

Combined typhoid fever and hepatitis A vaccine: comparison of immunogenicity and safety to concomitant monovalent vaccine over 3 years.

BACKGROUND: The safety and immunogenicity of Viatim, a combined hepatitis A (HA) and typhoid fever (Vi) vaccine, were compared with the monovalent component vaccines up to and 1 month after a booster dose at 3 years. METHODS: Healthy, adult volunteers were randomized to receive Viatim (group A, n = 179) or separate HA and Vi vaccines (group B, n = 181); subgroups were boosted after 3 years with Viatim (groups C and D, n = 56 and 46, respectively). Local and systemic reactions were recorded for 28 days postvaccination. Seroconversion and seroprotection rates and geometric mean antibody concentrations were measured at 14 and 28 days, 1, 2, and 3 years postvaccination, and 28 days after the booster dose. RESULTS: Local and systemic safety profiles were equivalent between the two groups. Immediate local reactions were infrequent (1 in group A and 2 in group B). Local reactions, consisting mostly of mild or moderate pain, were least frequent with monovalent HA. Antibody concentrations to both antigens were similar in groups A and B, in which HA seroprotection rates (> or = 20 mIU/mL) were respectively, 98.7% and 100% at day 28, and 99.1% and 99.0% after 3 years, achieving 100% after the booster. Vi seroprotection rates (> or = 1 microg/mL) of 85.2% and 84.9% after 28 days fell to 32.1% and 35.6% after 3 years, increasing to 67.3% and 69.8% after the booster dose. CONCLUSIONS: The combined HA/Vi vaccine, Viatim, had equivalent tolerability and safety and was as rapidly immunogenic as its component monovalent vaccines when given concurrently. A booster dose after 3 years significantly increased antibody levels with some evidence of relative hyporesponsiveness of the typhoid response.

A randomized, open-label study of the immunogenicity and reactogenicity of three lots of a combined typhoid fever/hepatitis A vaccine in healthy adults.

BACKGROUND: Travelers are often advised to receive both the typhoid fever and hepatitis A virus (HAV) vaccines, particularly when going to areas where the 2 diseases are endemic. Thus, combined administration of these vaccines could make immunization more acceptable by reducing the number of injections needed. OBJECTIVE: This study compared the safety profiles and immunogenicity of 3 batches of a combined typhoid fever/HAV vaccine administered using a dual-chamber bypass syringe. METHODS: This randomized, open-label study was conducted at 2 university-based travel clinics in Germany and Austria. Subjects received a single IM injection from 1 of 3 batches of the combined vaccine. Blood samples were drawn immediately before and 28 days after vaccination to evaluate the response to the 2 antigens by assessing geometric mean titers (GMTs) and rates of seroconversion and seroprotection. Subjects recorded all adverse events (AEs) occurring during the study period in a diary. RESULTS: Six hundred ten healthy adults were enrolled in the study. Twenty-eight days after vaccination, 90.6% of the study population had protective typhoid Vi antibody titers (> or = 1 microg/mL) and 100% had protective HAV antibody titers (> or = 20 mIU/mL). Seroconversion rates and GMTs were not significantly different between the 3 batches. There were no differences with regard to local or systemic AEs between the 3 batches of vaccine. There were no immediate adverse reactions (within 30 minutes of vaccination) and no serious AEs related to vaccination. Of 609 evaluable subjects (1 was lost to follow-up after the first visit), 555 (91.1%) experienced > or = 1 local reaction within the first 7 days after vaccination, mainly pain at the injection site (550 [90.3%]), but only 26 (4.3%) described this pain as severe. Vaccine-related headache and mild to moderate asthenia were each reported by 54 subjects (8.9%). Symptoms resolved spontaneously in all cases. CONCLUSIONS: The 3 batches of the combined typhoid fever/HAV vaccine administered by dual-chamber bypass syringe were equally well tolerated and effective in healthy adults, and did not differ significantly in terms of GMTs or seroconversion rates.

Safety and immunogenicity of a three dose regimen of two tetravalent live-attenuated dengue vaccines in five- to twelve-year-old Thai children.

OBJECTIVE: The safety and immunogenicity of tetravalent live-attenuated dengue vaccines after a three dose vaccination series were evaluated in Thai children. METHOD: One hundred three healthy flavivirus-seronegative schoolchildren ages 5 to 12 years were randomized to receive either dengue vaccine containing 3, 2, 1 and 2 log10 of the 50% cell culture infective dose, respectively, of the live-attenuated dengue vaccine serotypes 1, 2, 3 and 4 per dose (F3212; n = 40) or 3, 3, 1 and 3 log10 of the 50% cell culture infective dose (F3313; n = 42) or purified Vero cell rabies vaccine (control group; n = 21) given in a two dose schedule (3 to 5 months apart). A third dose was administered 8 to 12 months after the second dose to 90 subjects. Safety and immunogenicity were evaluated within 28 days after each injection. RESULTS: No serious adverse event related to the vaccines occurred. Most children experienced mild to moderate fever, rash, headache and myalgia occurring within 12 days after Dose 1 and generally lasting 3 days or less. One subject in Group F3212 had a 1-week dengue-like fever. Reactogenicity was minimal after Doses 2 and 3. Transient mild variations in liver enzymes and hematologic indices were noted mainly after Dose 1. After the third dose 89% of the subjects in Group F3212 seroconverted (neutralizing antibody response, > or =10) to all four serotypes, and all children in Group F3313 seroconverted. CONCLUSION: This study demonstrates a moderate although improvable reactogenicity and high seroconversion rates against the four serotypes of dengue after a three dose schedule of tetravalent live-attenuated dengue vaccine in children.